I would say your already overstocked with what you have. You're pretty limited when It comes to 6 gallons. Some people keep their one Betta in bigger tanks than that. But you would be better off with something like a Betta, and maybe a couple of Ottos.
However if you could get your hands on a 10 gallon it would open your options up a little more, or a 20 would be even better. Then you could do your Guppies and Shrimp if you wanted.