if you already have 3 clown loaches, get more! their a schooling fish that love the company of their own! they will eventually need to be moved into your 400g, but will happily live in a 90g for a while i believe.
theres tons of other options to add to your oscars as well, like more oscars of different colors, black belt, jack dempsey, clown knife, acaras, texas, geophagus (if your oscars are mild mannered) severums, fancy plecos and hundreds of other rare/fancy cichlids. 400g is alot of room, and with proper aquascaping with lots of caves, hiding spots, plants for line of sight breaks, you can have most anything that sparks your interest.
all but the most agressive fish would work in a community. some fish that arent such a good idea unless kept single or as pairs are red devils, flowerhorns, dovii, jaguars, butticofferi, beani...some of these fish MAY be able to work in a community set up, but its risky IMO. still amazing fish to keep however
keep in mind im only listing the common fish off the top of my head. theres thousands of cichlid species out there
