The number you keep depends somewhat on the species. With multies, a group of about 5 is what I'd go for with only one male and the rest female. You'll find they'll multiply though in no time
Also, as you already have some loaches in there, you need to consider how much space
they need and whether they'll be competing with the shellies for the bottom of the tank. What species of loach are they and how many do you have? Some loaches also won't like the hard water the shellies do.
Yes, they are only bottom-dwelling fish and realy, as they only ever stray a few inches away from their shells, they can be kept with pretty much anything that'll stay out of their way. They are also generaly too small to cause any damage to most fish so the only real issue is the difference in water parameter requirements (ie: shellies like the hard water/high pH of rift lakes which many other fish won't appreciate). I personaly have kept multies with danios and various (hardy and adaptable) barbs without any problems. They've also worked fine with various loaches, plecs and some other catfish. If it were not for their water requirements, I'd probably feel quite happy keeping them with things like hatchetfish as well (dispite their tendency to be bullied) because I realy can't imagine the multies going all the way to the surface to nip at them

I actualy did keep a few gouramies (three-spots, pearl and a few croakers - not all at once) in with multies at various points (though only temporarily because gouramies don't like very hard water) when I desperately needed a place to put the fish and the multies didn't touch them unless the gouramies went to
them first. Note, however, that I
am only talking about multifasciatus - there are many small shell-dwelling species and some are larger or more aggressive and maybe more liberal with their territoriality and therefore mroe likely to move further away from their shells and chase tankmates down.