Things I would like to do, tell me if its possible.

[Pollo2002]

1)I want to remove mirth and fortune spells from the game. Is there a way to make master genies not cast those spells in their repertoire?

2)I want to limit the number of active heroes players can have to 6 instead of 8, is there a way to do it?

3)I want to limit the number of slots for creatures heroes have. like having 5 instead of 7, is this possible?

[Arkas]

I'm almost positive you can take their casting ability away, or use the spell hooks in battle to make them have a different effect.

You'd probably need to disable the Inn function, possibly capture the mouse event in the town screen and run a script to check the number of heroes the player has.

[Salamandre]

Change their type when it is their turn, with BG0 trigger, and return original type in post trigger (BG1)