You pretty much hit the nail on the head. Each assist will have a bit of a cool down and it's length will depend on the assist. Since impmon will be the assist that you start with, his assist won't be too strong. I've planned for him to quickly launch a fireball in front of him, giving you a ranged option. Meanwhile, Angewomon would shoot an arrow after a bit of start up that would Pierce enemies and have more hitstun, allowing you to extend combos if you time it right. But like you mentioned due to a bit of a power difference, Angewomon's cool down would be a bit longer. So it all would depend on how you want to play.
It's all still in the concept phase, but this is pretty much the system I want to go with. I'll probably avoid go with healing assists tho since I don't want people to just end up spamming them constantly haha