How many fish have you actually got in the 2.5 now? This is getting confusing. Is it 2 bettas AND a dwarf gourami AND an ADF- or have you got more than one 2.5 tank? Sorry, I'm getting lost, too many different threads.
Anyway, IMO a 2.5 gallon is room enough for 1 betta, full stop.
Keeping a male together with any other betta, whether male or female may well result in bloodshed later on. Most people are very careful to separate them and only bring them together briefly for mating.
OR have I got the wrong end of the stick, and you mean, you don;t want to buy a betta for the tank, since you've already got these two other bettas in your other tanks?
In any case, the 2.5 is really unsuitable for most other fish. The advantage with bettas is they can cope with less oxygen because of their labyrinth organ, and they do not require a lot of swimming space because of the way they've been bred for centuries (and I believe they come from rice paddies anyway). Any other fish is going to be a lot more fussy; even guppies really need a 10 gallon to be able to display natural behaviour.
