Allowed sets are Black and White onwards which is the same as current official Expanded format You can find the rules, FAQ and ban list at the Gym Leader Challenge website
This Final Fantasy VI hero can become Esper Terra, a saga creature which lets you copy your other enchantments such as your other summons and she puts counters on them instantly so you skip ahead to their most powerful effects right away
You also want to make sure you don't dilute your main strategy and that the cards you include make sense
Some cards, such as modal double-faced cards or adventurer cards, can be cast as one of two different spells, each with different mana costs
Both Mega forms are temporary, lasting only during battle and requiring the Mega Stone to trigger
Even on his stats alone, a 5/4 for four mana with no drawback is extremely good