He doesn't seem that aggressive towards her to me. At his size and strength, he could really do some damage to her if he wanted to. If he is accepts tankmates and a little gill flaring and posturing are the limit of his aggression, I would leave it alone. Removing him and leaving him in a tank on his own for an extended amount of time may make it impossible to reintroduce him to a community setting in the future. My Midevil would have killed that entire tank in a few days. If you do move him, don't put that large Midas in a 55 gal for long. Are you trying to get the BP and Midas to breed?