Sorry, but there really aren't any other fish that would be happy in a 2.5. It's not just a case of size - you have to take into account other things like
- the other fish's activity level (for instance, neon tetras aren't big but they are active, and need a decent tank to swim around in)
- the bioload (a 2.5 isn't the most stable body of water in terms of the cycle, and you really don't want to overload the filter)
- the betta needing his own space. In a 2.5 he should happily explore every level of it and really enjoy the space as his own, and he probably wouldn't appreciate tankmates.
I really wouldn't consider any tankmates for him in less than a ten gallon. Then you could have:
- 6 pygmy cories
- 3 endlers livebearers
- 6 ember tetras or microrasboras
These are either/or options, though; you wouldn't want them all.
In the 2.5 I reckon shrimp would be feasible. The trick to keeping bettas and shrimp together is to give the shrimp heaps of cover. Java moss (impossible to kill) is great for that, and can just lie across the bottom like a blanket if you like. Eventually they will get used to each other, and should be happy. :) I have red cherry shrimp in my male's 5 gallon and my 16 gallon sorority, and the shrimp are happy and breeding. It took a while in the 5, but eventually my male ignored them and they got more confident. :)