beta's are also called fighting fish, and i'm pretty sure you can't keep them together at all.
a 2.5 gallon tank is pretty small too, should be really easy to throw off any kind of balance in there. Get like a 10 gallon one and start it off with a fish or two that's generally pretty tough to kill (cory catfish and swordtails are two that i've used).
http://www.fishforums.com/forum/ - two plus two for fish