No you cannot put a female in with a male betta, they will almost deffinitely hurt eachother. If you want a tank mate in a 2.5 gallon tank I would suggest maybe a shrimp. Thats about the only option I know of for a tank that size. If you wanted more fish you would have to upgrade to a larger tank like a ten gallon, then you could have some cories or other small bottom feeders.
Captain: Red and blue, still dont know the tail type in a 10 gallon divided with...
Neptune: white crowntail
Kokomo: cellophane veiltail in 5 gallon tank
Apollo: marble crowntail in a 5 gallon