Griz
Fish Herder
There are several shell dwelers that will do just fine in a 10 gal. Neolamprologus brevis,
Neolamprologus multifasciatus, Neolamprologus similis, Lamprologus ocellatus and Lamprologus stappersi.
The best bet IMO would be Lamprologus ocellatus. You'll be happy to know that they are also one of the nicest looking! There is a gold form that is sometimes seen and these can really be beautiful. You will however need to find an actual pair, or purchase several and allow a pair to form. Anything other than a pair has a good chance of ending in trouble. They are very feisty and should hold up against your petrocolas, but I'm doubtful that they would be successful in raising and young.
Thats by far too much activity for such a small tank, considering both the petros and shellies would be competing for the same living space. Now in a larger tank it could work and could be stunning if thought out at length.
You know what? You're absolutely right.

I stand by the recomendations for a 10 gal, but not with the petros in there. Especially with that many.