The "Walkthroughs" give you the general gist of what the factions do, but they've been expanded upon exponentially since then, so don't be afraid that's all there is, mechanically. You can have any combination of Runes you want in a Battlegroup, but if they are half, or entirely one faction, you get what is called a Faction Bonus (Shattered Peaks grants a small refund of Nora when you deploy Champions, Underdepths gives more Attack); typically, a "split-faction" bonus is just half of the full one, but I believe Savage Tundra gains one Attack for the full bonus, in addition to +10 Health instead of +5. (Even though I've played quite a lot, sometimes you do need to look up what the bonuses actually are—they're often not readily apparent.)
For packs, there was a change in the reward system years ago, and now you get "Core Set" Runes from each match. These are from the Pox Nora Release Set to Grimlic's Descent, and the game values them at precisely half that of the Extended Set. Extended is everything from Core, up until the latest set, which is "Expansion."
Generally, this just means that most people go for Extended Set Boxes, since you'll be getting Core Set Runes while playing, anyways. Boxes guarantee an Exotic rarity Rune, and they're ten packs for the price of nine (although I am seeing a discrepancy on the site, ATM). If you really want something from the Expansion Set—which is not available for in-game currencies—you'll likely be able to trade for it.
As far as playablity goes, literally ever Rune in the game, regardless of set, is good; some of them are just very situational.

Thank you for your help. I appreciate it.

So, it seems that I can mix and match the runes. How about the actual champions? Like, can I put in a couple dwarves annnnd a couple skeletons together? And is there a specific extended set I can purchase, that comes with champions, or is it all random?

I also see that you can purchase pre leveled built decks. I guess it's best to play through the walkthroughs of said factions before doing so, but I assume that is a decent way to go. I plan to just play on my own and Skirmish stuff. Not really online against people.

Absolutely; any configuration of Champions. Typically, races and classes work as themes, but most people seem to use meta Battlegroups which just have whatever is "good" in the meta. Some races work best when they're together, such as Jellebrium, for Forglar Swamp, Minotaur, for Underdepths, or most notably, the Moga and the G'hern pairing in Shattered Peaks.
Most-but-not-all sets contain at least one Champion of each rarity for each faction, and there's a very good balance of Rune types, in my opinion. If you're playing on PC, you can use the Rune Forge on the website to craft whichever Runes you desire, save the "Limited" Runes, which are all event or a few Campaign rewards, and the Expansion set. Otherwise, each pack will just pull randomly from applicable sets.
I also only play against AI, and I'm a F2P player, so I just started with packs. I remember the premade decks being good value for their rarer Runes, but I can't, from memory, confirm that's still true—but it should be. So if you got some nice Skelebros from packs and—I went to double-check the name of a Rune and saw that the decks have changed. Either way, 5,000 Gold is an amazing price, so yeah, go for it. And they're all thematic, too, which is great.
Themes are the funnest way to play, in my opinion, and they're easy to build. For example, if you have a Battlegroup built around dealing high electricity damage, you may have the Spark Crone, which causes everyone who receives the electricty damage-over-time effect to become Cursed, and if you have a lot of area-of-effect damage, equipping a Lightning Rod to an enemy Champion in the middle of a group will deal 80% of that potential damage to that one target, instead (or you can use it to burst-heal something with Electricity Eater, which would be slick).

Thank you again for all the info - I'm really enjoying it. And very good to know I can mix them, wow. I am really liking the ranged Elves, but I did just get stomped by some dragon beasts LOL. Yes, on PC.

So, the deck I just recieved of the Elves by beating that kthindr forrest walkthrough, is not the entire deck/set? I assume I get a whole lot more Elves, buying the deck with money and there will be extras I don't have now?

Well, there are about 76 non-Demon Elves in the game (the Underdepths' Deep Elves are both Elves and Demons; they also have a non-Demon Elf), spread across all sets. The Rune Checklist will show what's in each set and how many of them you own. Some of the sorting categories are from very early on and are outdated ("Bird" is not a race), but it's very good for getting the gist of what's available.

Buying the "Dancers in the Snow" deck and sacrificing the runes gets you 2156 shards. This is enough to make 2 legendary runes and 1 epic rune. You can buy it several times so once you find what you like to play you can just buy it to craft whatever runes you want.