The main trick is to avoid anything that would share the kribs territory or be seen as a threat or competitor. This mainly includes bottom feeders and other cichlids. Anything else, within reason, is fair game. I wouldn't go with a small and timid fish like a neon because they may feel the stress of the cichlid aggression even if they aren't being targeted, but it seems to work well enough for others. Any characins will appreciate a decent shoal for security. Rasboras will work too.
When it comes to cichlids sometimes things will work, sometimes they won't, so always keep your eyes open and try to play the odds in your favor by avoiding risky livestock. Little peaceful catfish like cories may work, and I must admit has worked for me in a krib tank, but are best avoided nonetheless.