Its very tough. Watched the video. Obviously, the best choice is not an easy one. Either you move him, risk the Fun sized Kuma's health and risk tank mates safety, or you take a gamble and do not add the stress of moving him back. Yes, I just lost my two alleged Fila's. Over summer, I added the much smaller one to my pond. He displayed very, very similar behavior as to yours. Swam very high in water column and was all banged up, and lost streamer on caudal and lost whiskers. His caudal was almost entirely bitten off by one of two suspect, Wallagonia micropogon, or Brachyplatystoma filamentosum. I suspect the latter. Either way, had time to bring him out, put him back into a 300 by himself and he healed. Never have seen anything like that. If there's a message I want you to get out of this is don't give up hope yet, as they can be surprisingly hardy. I hope this resolves itself and is just an extreme pecking order readjustment.
-mike