A common Pleco would be happy in a 90 gallon. Something like a school of Ottos probably wouldn't last two nights against the Bichirs (and possibly the Severums too). You're right, you need something tough, and a common Pleco seems like the best choice. Most pet stores should have decent sized specimens (4"+) that people have turned in because they got too large. Plecos tend to get lazy as they get larger, but in a tank that size one may feel more inclined to move around and do some cleaning.