Do any of these fish do well in a 2.5 gallon?, besides the betta
The least killifish (Heterandria formosa), Pygmy killifish (Leptolucania ommata), and bluefin killifish (Lucania goodei)
The least and pygmy killies you could keep a couple pairs in a 2.5. You could keep a single bluefin killie in one.
Also you could keep a pair of one of the pygmy sunfish (Elassoma sp) species.
You also might could get away with a couple of endler's livebearers, but I'm not sure since I've never worked with them before, so I don't really know their requirements.