Kribs form breeding pairs and will attack any fish in the community if they are spawning. I'm not sure if it's just me, but they have attacked even the tiger barbs, which are supposedly good dither fishes :S
If you put a krib in a community alone, it depends on the individual if the krib will be aggressive. I have a male that isn't good in a community, while one female on chases the top-level fishes from time to time :S The other female doesn't chase any fishes around. As I said, it depends on each individual fish.