I think you could get about 3-4 rams, like a male and 2 females, or you could get shelldwellers if you have hard water. There are some apistos you could get, but you would need to stock it like with the rams.
If it is like most 15
gal tanks it is relatively tall with a small footprint and therefore not really much room for two species of cichlid.
I have tried to keep a pair of kribs in a 15gal and found it to be a bit cramped for them, so I'd recommend against those in anything less than a 20gal, although they are really one of the best kind of cichlids if you are new to cichlid keeping.