They are naturally territorial fish and will clear other fish from an area in order to lay eggs and bring up fry and will defend their eggs and young from potential predators (which means any animal big enough to fit an egg or fry in its mouth).
Other fish simply scatter their eggs so don't require this behaviour which takes up energy.
If you asking what could be kept in their territory I suspect the answer is nothing ideally. Anything weaker would be chased (with the intention of chasing it out of the territory which is impossible in a small aquarium). Anything stronger would stress the kribs out.
Mine bred in my six foot tank and required 4 ft of the tank in which to breed. The rest of my community fish occupied only the 2ft remaining. They were hard working parents and worked brilliantly as a team.
If you only keep one I would recommend a male, females can be a bit. I have kept a pair in a community tank and they were good until they breed and then all fish were a threat, they are excellent parents and will protect there fry at all cost.