I have a couple different types of Apisto's in different tanks and you could get by with putting one pair in a 15 gallon, but that would be the max. I agree, the Cacatuoides are beautiful (I have some on my "to get" list this summer at the Cichlid convention), and I think you'd love them.
IMO, Kribs are a bit too big for a tank that size.
If you really want a small group, the Cherry barbs would fit the bill very well. I have some and they are just firey red.
I've never kept Sparkling Gourami's or Rainbows, so I can't really comment on them. I know many people who do really get hooked on them once they get them, though.
