Kribs would work fine in this setup....Blue Rams would not, as the above member stated, they are not happy when kept with boisterous fish. Bolivian Rams, on the other hand, are quite a bit more hardy and grow larger than Blues. Other suggestions....most Geophagus species are also on the non aggressive side, just keep in mind they like to dig and a few become a good size. Severums are also possible in this respect. A West African dwarf cichlid (not from the Rift lakes) named Anomolachromis thomasi is another good choice. They stay small but can definietly hold their own. In any case, make sure you have plenty of hiding spots in the tank for the newcomers to hide.