The most common rule for tank stocking is no more than 1 inch of fish per gallon...so if you have 2 ghost shrimp, you've already used up about 2 inches (2 gallons). You can safely put three 1 inch fish into the tank..or two 1 1/2 inch fish. It's not large enough to breed anything. The problem is that water can only absorb so much oxygen over a certain period of time. Tanks with more water surface area do better than tall tanks, as a result. If the fish don't have enough oxygen, they won't survive. Bubblers really only circulate the water so the oxygen is evenly distributed, they don't really add much oxygen--it comes from the surface.
You may be able to keep 3 neon tetras, or a couple of zebra danios. I would not get guppies, as they will breed...and then you'll have the problem of an overstocked tank.