I’ve had luck with black convicts givin the convict is big enuf to not get ate. A Marble Goby is a nother good one I’ve kept with em. I fed mine a mix of hiarki massivore an feeder goldfish from time to time. The goldfish is cool cause the marble goby usually will bury themselves in the sand an explode out when the goldfish gets to close an chomp!
Most of the time poly on poly aggression is dominance stuff and should settle down. As for how small with yours at 6-7 ive put as small as 4" long with my 8-9" dels they were the biggest the last time I added a poly. Dithers Congo tetras, Denison barbs, larger body tetras like bleeding heart, diamond, or shoot lost my train of thought on those, giant danios any of these could work and shouldnt get eaten.....i say shouldnt but you need to be prepared cause they all could. Most of the larger barbs and alot of barbs in general will destroy your plants.
Congratulations on the new move! Last Saturday I introduced 2 new bichirs in my tank as well, a 12 inch sp Koloton & a 14 inch PBB. A ton of slime coat was shed as well - but on Sunday both already started eating. Did a 50% WC yesterday, and fed a little (everyone accepted food), and did a smaller feeding today. PBB has not been eating as much as I expected him to, but that's alright, she's probably just settling herself in.
Zero to very minimal slimecoat shedding after the WC yesterday. Keep us updated, and don't sweat it!