Which evocation cards should we include in our game?

[7h0r]

Do we use the evocation cards that come with the schools in the current library, or can we use all evocation cards across all the game content that we own?

[Beto_BringerOfOrder]

There is no rule about it, even we never thought of creating one to give you the freedom to do as you wish, there is no limit to what you can conjure up.

The limitation is only in the material you possess!
I would tell you, if you think it is unbalanced just use the summons of the decks in play.