I just converted my 20 long to a krib tank, so my answer is not based on long-term experience but from what I read and the advice of others.
Other kribs with a breeding pair - 0 (your tank has the same footprint as mine, and kribs live in the bottom/middle zone so you really don't have the extra space for another krib or two)
Other fish - top/mid-water tetras (congo or their smaller relations), most SA tetras that are on the tougher side, just about any barb. Also you could have killifish up top, and I was recently told an African Butterfly fish should do OK since they hang on the surface.
Most bottom feeders won't work. I might try upside down cats since they're from the same part of Africa as Kribs and spend a lot of time up top, too. Bristlenose or rubberlip plecos work very well & stay small enough, too.
HTH,
Eric