Ok, my opinions on shellies from personal experience...
Multies make a nice colony. They do not like substrate and will try their hardest to get rid of it, meaning piles end up in the corners, back, or front of the tank. It's actually rather amusing and the best solution is obviously to not give them much to work with. I have maybe a half inch of crushed coral in my multi tank, the main purpose for having it is to buffer the water. Unlike other shellies that do fine with 1-2 shells per fish, multies like lots and lots of shells, I think my 10g has ~60 whale eye shells. And those will get rearranged too. I started out with 8 adults in a 10g. I've not kept similis but apparently they will establish a colony, although they're apparently more agressive than multies.
So far I've found brevis to be my most prolific and best parents. Very dedicated couple, both to each other and to the fry, at least until another batch comes along. They're a bit larger; with a 29g you could have 2-3 pairs.
My stapps are number one for looks and personality; unfortunately I've yet to get fry from them. I have 6 in a 20 long with juvenile calvus and the stapps have made it clear where the territories are. Like the brevis, you could probably have 2-3 pairs. Occies and speciosus ('black occies') are supposed to be quite agressive. If you can find them, hecqui might be a good choice if you plan to have fish other than shellies in the; it seems because of their size and personality they do well defending themselves-and fry-against other types of fish.
No, I definitely wouldn't mix shellies...even if you aren't interested in breeding there would be too many agression problems between the adults. A 29g is high, you could probably get away with a few julies along with the shellies. Calvus or comps are also said to work, that's what I have going on in my 20g but like I said all the fish in there are young so the situation may change as they grow.
