RE: How do I now if (EX) cards fit?
If by "fit" you mean "work together", then I can offer you this piece of advice.
I think part of the beauty of Pokemon is that you can run whatever you want. But there are some stipulations. For instance, you can't run a Xerneas EX, M Blastoise EX / Cobalion EX list unless you have a good, binding Pokemon to rein them in together as one. For instance, Aromatisse. Meaning, you can't run a bunch of cards that don't have synergy, so above all else, you can run whatever you want, just as long as it has synergy.
Take Zekrom EX for example. Strong Volt discards two energy attached to him as the recoil from that attack. You're doing 150 (or 170 with a Muscle Band), but you discard those energy, and that means you kind of have to fight your luck to attack with Strong Volt that next turn. Nasty side-effect, no? So, looking at the card, you see that it takes two lightning and two colorless, and the wording of Strong Volt means you can discard any energy, not just lightning. So, you begin to look around for cards that either attach energy, or move them about.
For instance, Aromatisse can move a certain type of energy around, so after you discard those two energy, you move two more up and deal another 150. Or, as another example, Blastoise or Emboar can attach their own certain type of energy from your hand to one of your Pokemon, thus using those energy to discard with Strong Volt. Remember, colorless energy is filled by any energy type, so there's not much to worry about with adding on support.
Another example. Skarmory EX has Tailspin Piledriver (an awesome attack name to say, by the way) for 80 base damge, that reads "If the defending Pokemon has any damage counters on it, this attack does 40 more damage." and as such, bringing the damage up to 120. Now, if you play something like Darkrai EX, Registeel EX or the much easier to obtain Kyurem from Legendary Treasures that puts thirty on benched Pokemon, you can attach a Muscle Band and deal a resounding 170 total, KOing most played EXs.
I guess what I'm saying is that you can play whatever you want, just as long as you know two cards work together with a synergistic bond, if that makes any sense. One should cover another's weaknesses, enhance the other's playstyle or provide a neat ability to power up you Pokemon quickly, make them deal more damage or make them survive longer. It's a sheer matter of preference, but I hope I've made sense.
However! Not all decks have to revolve around EXs! There are plenty of really good stage 2s you can play, such as Garchomp and Empoleon, the former being one of those "out of style but kind of okay when you think about it" cards and the latter actually taking some City Championships.
Also, please keep in mind you don't have to actually
have the cards to playtest them! There are really good proxying sites out there that give you a print-out of that card where you can put it in a sleeve and play it as if you own that card. You can't use these in tournaments, but let me tell you: they are utterly fantastic for playtesting, especially when you have twenty to forty dollar cards like Virizion EX in the format. I mean, if you go out and spend all that money, only to have the deck fall apart, you would have just wasted a great deal of money and time on something that you, what, played all of one time, found out it was bad and stowed away for evermore?
Anyway, I've rambled on long enough. Hope you make a good deck, and I hope you have fun making it!
