well 16 gallons isnt much to work with, but it would make a nice nano. Are you planning on corals??
I would say that a pair of clowns would be fine, but you really wouldnt want much more then that. There are a lot of nano fish, mostly gobies that are better for smaller systems.
Cardinals are okay but they get larger around 2 inches and can be rather active. A little bit more suited for a larger tank.