Now I had read somewhere (and I wish I remember where, but I think it was The Cichlid by M.E. Sweeney) that target fish are sometimes a good idea for breeding pairs of Cichlids (to maintain harmony by providing a conduit for behavioural displacement), but they should probably be something more expendable than Congos. Plus, I am not sure how aggressive Kribs get when breeding, so they might be okay without them, and, indeed, if they don't kill the target fish, said fish will snack on little Kribs.