I don't grow to sell, but I can imagine that business would probably be better if you specialized in one particular "type" of plant. I say "type" because you could collect every species of that type including the harder to find specimens. For example all rhizome plants like Buces and Anubias. Or collect and grow just mosses and liverworts. Or all the swords and other leafy plants that propogate with runners. Unless you have the space for multiple, I think this would probably make you more money, if only for the reduced overhead involved.
To answer your other question, it probably doesn't matter what you grow them in as long as you're physically able to carry out your routine maintenance and pruning/propogating.