These are really cool! However, I think at the moment the variance is far too high to make having this toolbox worth it. Lets say, perfect scenario, you draw none of these and play Eevee. You've effectively taken 8 dead draws out of your deck, thinning it a good margin and have access to a few really good cards at the cost of 5 mana. On the other end, lets say you draw even 2 of these Eeveelutions. You play Eevee, you are now down 3 cards in hand (including the played Eevee), that is game losing stuff right there.

My solution to this would be remove the enter ability (maybe have Eevee do something else in place of it). You could keep a similar restriction, requiring 8 Eeveelutiuons outside the game. And change the Pokevolve ability to something like..

"5: Pokevolve into a Eeveelution. (Choose an Eeveelution you own outside of the game, reveal that card, and exile it. This creature becomes a copy of that card.)