This has probably already been addressed but didn't see it quick in a search. Just few things I've noticed.

Companions do not use power from fusion cores while in Power Armor.

To remove a companion from the power armor simply speak with them and ask them to get out of it.

Danse has a fusion core in his inventory for you.

To repair power armor damaged on a companion simply remove it from the frame, take it to a bench that has a frame in it and you can repair any armor inside your inventory and on the frame in the bench. After repairing, just trade with your companion and move it back. Make sure to re-equip it on your companion. No need to have them get out, unless you have no other frame. But there are plenty of suits around.