The main difference between custom units and vanilla units would be the fact that vanilla units are significantly less detailed, wearing only a shirt, a pair of pants, shoes and armour. Play around with the facial features or clothing and use the colour slider. You'll notice it'll look cooler.

Pretty obvious aye.

This is important, you probably wouldn't want someone wearing a black shirt to have a pink beard, you only see those in anime or something. Make sure the colour of whatever the unit has on to suit the rest of the outfit, otherwise it'll look like it was sprayed by paint.

It ain't hard to do either.

I actually had some trouble with this.

Remember that you can't increase player damage in custom battles, so unless you want to fight the terminator or something, don't give it a ridiculous amount of health.

I'll give some examples cuz the game is still loading. (Yes, its a lot of mods, 44 pages in fact)
A basic unit should probably have 100-600
A heavy unit or an elite unit should probably have 600-1000
-If you wanted a challenge or something, scale these up.

I dunno, its more of personal preference really.

It just depends on your other custom units or how strong you want it to be, so I'm not gonna go over this in detail.

These are just cosmetics with some perks. Like the cosmetics, experiment with them a bit, play with the colour as well. Just re-read the cosmetic part of the guide and replace anything to do with cosmetics with the word armour.

No, I didn't put any effort in it.
If this guide gets over 2 stars, I will be very disappointed in all of you.
As a reward for reading this trash, I'll introduce some half life culture to you. you'll probably be traumatised for life after seeing this tho but if I have to suffer, so does everyone else.