id go another bichir or 2 or 3. They do well in groups. Wouldnt go featherfin cat, i have 6 and would think a bichir would get beat on, theyre bulky things, also ive read a worrying amount of stories of featherfins beating on plecos, they just dont like em for some reason, maybe its a territory thing. rafael cats should work my bro has 3 huge ones like a foot long in his angel and sev tank with plecos and they are very peaceful.
Ah just realised u went for a featherfin, I can safely say that these are very much gregarious fish, in my experience they are very much like massive upsidedown catfish, they act the same and huddle together, chase each other and just generally seem to enjoy each others company, i never see one out on its own, its all 6 or nothing. Maybe these reports of extreme aggro are due to single specimins being kept. I mean the smallest of tetras can rip fins to bits if kept by itself, put it in a group and alls good! Beware they grow extremely fast!
Just keep a close eye on the featherfin, remove it at the first sign of trouble or add another 3 or more of them. More than any fish ive kept these guys seem to need a pecking order, a hierachy. They display and fight for dominance and the big one usually ends up ruling the roost keeping the others in check, occasionally the big one may get challenged and the order changes but these fish seem to need it or they just go on a killing spree once they realise just how huge and bulky they are, one of mine side swiped an adult krib and knocked it dead! (On that note i hope you have heater guards, no fun picking glass out the sand!)
This sums up pretty much what ive read about em in the forums... http
/www.practicalfishkeeping.co.uk/content.php?sid=4506 saying that ive had absolutely no trouble with mine but ive always had 6 from when they were 2inch pups, no joke they were 6inches within a month, a year on they are truley monstrous 10+inches long and 6inches high when thier fins up! All id say is keep a close eye on the bichir and pleco, maybe even the knife, this fish loves its caves and doesnt like to share.
#40## i hope i havnt scared ya, great fish, i love em. Just from my personal experience i cant stress how important them being in a group seems to be, think of them as giant upsidedown cats or giant corys, you just wouldnt keep em by themselves.