If you have a pair of kribs and a pair of rams, they should be OK in a 55g. The big trick is to have PLENTY of "groundcover" in there so they can stake out their own territory and have plenty of hiding places. If the kribs choose a territory in the middle of the aquarium, re-decorate and move their spot to the side. If the kribs claim the center of the tank, any other bottom dweller could be toast.
As for what else works....well, pretty much anything that's not a bottom dweller and is fast enough to get away from territorial kribs and hardy enough to withstand a nip or two if the kribs happen to get them.
A school of rainbowfish would be pretty.