When choosing the traits for SCW, you need to consider it's characteristics. It's quite different in design from the main campaign and differs a lot from 1939-1942 grand campaigns.

First of all, you need to remember that you get no infantry of your own for the whole campaign, so your infantry units will get no XP and can’t level. You will have to buy auxiliaries during every battle.

Tanks you have are very early models, and when coupled with the fact that they are inexperienced, they can get hurt easily and don't do much damage. You will need to make sure you only use them in the open terrain, hunting for other infantry, scouts and artillery. Use bombers rather than tanks to fight other armor.

You artillery is pretty bad and your strongest point may be your expensive airforce.

The only positive is that your enemy isn’t much better than you.

Advantages:

Infantry general 0/5 – As you will not get your infantry, only auxiliaries in civil war it is worthless. Gets much better in future campaigns.

Panzer General 4/5 – Great overall, not that great in Spanish civil war, as tanks are weak, but still good. Remember that your units will carry over and will be the backbone of your army so field them, get them awarded and experienced.

Industry Connections 0/5 – I consider it worthless, as you never get enough to field more than one unit, and once it’s damaged you only have a very limited number of replacements. Also, it doesn’t happen often.

Liberator 1/5 – mediocre – If you play well, without needless losses, you will never starve for prestige and will end up having a lot more than you can spend.

Deep recon 1/5– It’s a good trait, but hardly great. But it becomes very good if you play with shroud on ("shroud" is a difficulty option separate from generic "fog of war" and it makes the whole map blacked out).

Operational initiative 0/5 – worthless

Master of Blitzkrieg 5/5 – best trait you can get. Even with SCW bad tanks, being able to move quicker and catch the infantry/artillery in the open is how you want to play.

Battle academy 2/5 – Somewhat useful trait. You start very green, but hardly great, you will end up capping exp of your units anyway. You will get more experienced units in earlier missions which will make it a bit easier. There are better options though.

Auxiliary 0/5 – Actively avoid it; Thing is, you should avoid auxiliaries unless absolutely necessary (sadly in civil war you are forced to pay for infantry) for three reasons: they cost prestige, steal xp from you units, don’t carry the exp over. Even considering you need to buy auxilliaries in this campaign, only buy as little as you absolutely need.

Trophies of war 1/5 or 4/5 – If you’re sure you know how surrenders work, actively seek them, then it’s a very good trait as communist tanks are awesome compared to yours. If you’re not and are able to get occasional surrender here and there.. don’t take it. If you play with The Collector on, where it becomes a must, you won’t have enough surrenders to make it worthwhile.

Deadly grasp 2/5 – don’t underestimate it, these penalties hit hard if you manage to surround.

Flexible command 1/5 – extremely situational, can very well finish the game without splitting

Killer team 5/5 - great, but depends on heroes you get. Heroes can make your campaign (and future ones) easier than having another unit in the field.

AA veteran 3/5 - good, but hardly breathtaking; I usually play with one AA as fighters are more versatile.

Perimeter control 2/5 (3/5 if with deadly grasp) – great combo with deadly grasp, you can surround without being surrounded a lot easier.

Meticulous planning 5/5 – Great, can’t overstate how it helps in setting strategy. It’s the only way to play with fear of unknown disadvantage

Force concentration 0/5 – Worthless in Spanish civil war, really good in later campaigns.

Terrain expert 0/5 – not worth it; If you’re in a river and someone catches you with pants down, it’s your fault. You are almost always on offensive so sitting in one spot for a few turns works only in two battles (granted, one of them is Ebro - the hardest one in campaigns until Grozny).

Aggressive deployment – 3/5 Almost as good as master of blitzkrieg in campaigns later than 1939; You never have to bother with mobile artillery again and can only focus on godly 21s. Your infantry moves and attacks as quickly as tanks do. Not as useful in civil war, but still good.

Old guard 0/5 or 5/5 – Must have for ironman; useless otherwise. If you can load/save and undo and still lose units at the end of the battle, try figuring out what you are doing wrong instead.

Inept logistics – One of the worst choices there is. You will field 2-3 less units in every single battle
No overstrength – can live with it, but pretty punishing. Overstrengthening your most elite units can allow you to break through defenses in no time with fewer loses than fielding two mediocre units without OS.

Denied air force – just don’t do it in SCW; Airforce is your main advantage. It’s possible to use AA and artillery instead of fighters and bombers but extremely painful, slow and takes more core slots. Bombers are much better against enemy armor than anything else you can field in Spain.

Denied artillery – actually not nearly as bad. Artillery in civil war has low range (except for 201s you can get in two scenarios), mediocre effect and is nowhere near as good as in 39-42. Enemy’s AA isn’t deadly so you can just field more bombers instead.

Green army – It's an extremely bad choice for SCW, very punishing. You may be forced to fully retire some units from battlelines not to lose them or lose their xp; Your units are green and will be taking losses. It's not a 1940 army which blitzes over enemy panzer corps losing 1-2 strength.

Delayed reinforcements – Almost the same as above, only worse.

Retrograde – I haven’t tried this, but doesn’t actually seem bad. For most of the campaign you play the same unit types and upgrades are rarely gamechanging. Sometimes you get chance for new equipment for command points and you can always capture enemy’s equipment.

Poor maintenance – doesn’t happen all that often but sometimes catches you just in the wrong moment. If you lack points but not luck, choose this.

Chaotic fire – It’s OK to take this in civil war, would be more punishing later. It will be a while until you get steamroller and even longer until you can overrun more than twice in a turn, and that’s pretty much the only case where it plays a role – just don’t get surrounded.

Inefficient supply – same as above, shouldn’t change much, just don’t take this and chaotic fire at the same time.

Poor ground control – don’t take it; enemy will go right through your lines and pick artillery or break out of surround.

Bad luck – safe to take, sometimes makes it a bit harder, depends on random factor % you chose in advanced difficulty options.

Trench slog – A really bad option. It’s actually much more punishing in civil war than later on when you will get multiple options for dealing with entrenchment.

Slow modernization – When in doubt, take this; you can wait a bit with upgrades, and you usually won’t get all the equipment at the same time anyway. You can also take it if you want to catch enemy units and field them instead, they are better than yours.

Fear of unknown – Changes game a lot; you will need more scouts or scout planes, slows your map progress, but if playing ironman you would be moving carefully anyway.

Arrogant – In theory doesn’t change any mechanics, but if you can’t see the predictions you have to consider every combat very carefully, checking for entrenchment, artillery support. Gets very tiring quickly.

Force dispersion – Not all that bad in civil war but never take it from 1939 on.

Ruthless – If you will never use captured equipment and don’t care for prestige (you shouldn’t, there’s plenty unless you play at hardest settings – and then you wouldn’t be reading my tips), choose this.

Slow reaction – Never choose this; it’s a nightmare of having every enemy using with a hero with “No retaliation”. Just consider the fact that a 15 strength tank with steamroller, and heroes with 'no retaliation' and 'ignore entrenchment' can clear whole enemy city filled with dug in infantry.

I hope it's useful to someone and that you will enjoy the SCW as much as I did!