In a 5 gallon or less, you really shouldn't get any tankmates except for a snail or some shrimp, for two reasons. Firstly, a betta may not appreciate having to share that small space. Secondly, a five gallon can only take so much in terms of bioload (how much waste is created relative to how much the filter and water changes are removing).
In a ten gallon or up, you could consider tankmates. Personally, I wouldn't go for glofish in anything under a 20, and I'd think twice even then. Glofish are a form of danio and can be nippy, plus their extreme activity can stress a betta out. They also do better in slightly cooler water, whereas bettas enjoy warmer. You definitely couldn't get the necessary school of 6 into a 5 gallon.
For your 5 gallon, look at a snail or some shrimp. There are a lot of shrimp species available, but the three main "beginner" shrimp are ghost (clear, good scavengers but they do have claws), cherry (scavengers and moderate algae eaters, no claws, peaceful and easy to breed, pretty) and amano (slightly more brown than ghost shrimp and great algae eaters). Sometimes shrimp will end up as nothing more than a betta snack, but some bettas will ignore them. :)