There is actualy quite a bit you can do in a tank that size. The alto's you mention are great fish and for the most part will go with just about anything. The dwarf julies (Julidochromis dickfeldi, ornatus, transcriptus) would also make a good addition to the tank.
Or you could also do some of the smaller shell dwellers.
As for the numbers, that is going to depend on what age most of the fish are when you get them. You may start out with 6 julies for example, but once they mature and start to breed, you may find yourself with only the pair left. The alto's really aren't that much of a concern in this as it takes a very long time for them to mature. If you want the best chances of the julies breeding then I would get at least 6 fish to let them pair off on their own, just be ready to move the ones who have not paired up to either another tank or take them back to the store. It may work, but in such a small tank it would be a lot of stress IMHO to have more than one pair of julies breeding at the same time.
As far as the shell dwellers go, search around on the net some about the different species and see what strikes your fancy. They are a really neat fish and out of all the species that are available in the trade I'm sure you'll find one or more that you really like.