anything that'll hide well or is armor plated like the doradidae family. you also need to make sure they aren't going to be small nough to be eaten. some pimelodidae family has been successfuly kept with cichlids too but most are far too big for a 90.
yeah anything from the doridae family would be good. pim Blochii would work well too, a good sized catfish that is semi active and not too large for a 90