The reason they are not getting along is due to territorial disputes. The Kribensis Cichlid is VERY territorial (especially during mating/breeding) and do NOT tolerate intrusions. Judging from your photos I would say that there are not many (or any at all) hiding spots. Cichlids in general thrive in environments that are riddled with caves, cracks, and crevices. This allows them to setup their very own territory away from anyone/anything else. Hiding spots also cut down on aggression and stress since the fish are not forced to be around each other 24/7. Nine out of ten times (if compatibility is not an issue) aggression in the aquarium is caused by a lack of privacy and personal space.
If you do some light research you can find out quickly that one Krib requires a MINIMUM of 30 gallons. Although on paper 26g seems very close to 30g, in actuality 4 gallons is a lot of space when dealing with smaller tanks. This is not so much the case with larger tanks (ex. 75g compared to 80g). Also, I am sure your female in your 5 gallon is absolutely stressed to the max right now. Try to remedy the problem or re-home her ASAP to avoid illness or possible death.
IMO if I was in your situation (and resources willing) I would upgrade to around a 55 gallon tank. That way each Krib has PLENTY of room. Make sure you provide adequate and ample hiding spots or else you will be right back at square one. In a 55g you could even add a few compatible tank mates or who knows, maybe the male and female will breed (if correct conditions are met). For right now however, a TEMPORARY and cheap fix would be to purchase a few caves from your LFS. You can even make them yourself out of pots and PVC tubing (just make sure they are aquarium safe!). At least this way they can have some cover and shelter until you decide a solution.
Hope this helps!
Kentaaa