Actually, you could probably get a good breeding colony of the 3 smallest shellies in there, or six of the larger ones. Research carefully, though - Neolamprologus ocellatus (I think) you might be able to get two pairs if you put a tall rockpile in the middle of the tank, but probably not. They are the one that sticks out as being particularly aggressive when breeding.
Based on the article in TFH last year, you could probably get a half dozen dwarf puffers in there if you planted it well.
Thinking back, If you were only going to put in one species, you could get 6 of any but the largest cories in there with a fair chance of fry if you treat them well. (Scleromystax barbatus
, Bronchis britskii, and Corydoras negro would be too big, the rest would not.)
Raphaels are too big - they get up to 7".
A Clown Pleco
would work, but 1) you'd rarely see him unless you used very dim lighting and a ton of driftwood as cover and 2) driftwood becomes obligatory. Rubbernose/Bulldog plecos would also probably work, but 1) they like a good current and 2) some of the species prefer cooler water. Upside Down Catfish
(Synodontis nigroventis) should work. They really should be in a larger tank and in a shoal at that, but 5 in a 15 with nothing else might do it.