Aaragonite is better then crushed coral because it's finer, with no sharp edges. It will help a bit to buffer the water.
A PH of 8-8.5 is ideal. They can live in any water above neutral without a problem, but need the higher ph for breeding.
In a 10 gallon you'll need to do a species tank, there isn't enough space to start mixing Tanganyikans - they are very territorial.
How many fish and how many shells depend on what species you go with. First find out what you like, or what you can get your hands on, then work on numbers and tank decor. Some species are pairing, some are harem and some colony breeders, and they range in size too.
Multifasciatus are great because of how small they are, and how well they get along in a colony. WIth these guys just get as many shells as you can get your hands on and throw in 5 or 6 fish. Males are nearly twice the size as females once they grow, but if you're buying juveniles you won't be able to sex them. Since males will co-exist, you don't need to worry much about it. Some danio's or something that sticks to the top will work, but otherwise give the shelldwellers the full bottom of the tank.
