Too small for anymore fish unfortunately but a mystery snail would get on just fine with your betta. If algae is an issue Nerite snails are usually good algae eaters and are rather pretty which you could get instead of the mystery snail perhaps.
Mystery snails are pretty big waste producers so remember to keep up the water changes. :)
I agree that a bigger tank would be better. Does the 2.5g have filtration and a heater at least?
My favourite cleaners are amano shrimp. Busy cleaners and fun to watch!
Might be pushing the limit though if you add a couple to such a small tank.