It's not a melanochromis, it's more preudotropheus like but I bet it's a cross / hybrid. It appears to be a male, it's anal fin is elongated and has egg spotts.
Good possible canidates for additional tank mates would be auratus, chipokae, other zebras (red, blue, albino, cobalt, etc.) Venustus, polystigma, livingstoni, fus****einatus, trewavasae, fullaborni, tropheops, elongatus, demasoni, and many more. You got tons of choices and colors to choose from. I would suggest to get fish only from lake Malawi. Don't get peacocks (alunacara species) as they are kind of wimpy compaired to mbuna.
The shells would be fine, will not hurt anything at all.
Joel