ummm ok there's a few things already wrong here, male and female fighters are not kept together unless for breeding, as you found out before one will eventually turn and kill or badly injure the other.
2 gallons is VERY small for what you have there, I have a 3.4 gallon tank which I'd never keep more than about 5 guppies in or one male fighter.
If you can measure the tank with a ruler or tapemeasure there is an aquarium calculator on this site which will work out your tank size for you.
I'm thinking your tank must be bigger than 2 gallons because you'd see there wasn't enough water for fish in it.
Barbs are nippy fish regardless of numbers and are not good betta friends, what sort of pleco is it?
Mollies and bettas are ok, guppies are not normally advised bcause bettas can mistake them for rival and injure them also but if you've had no trouble there, then that may not be an issue.
If your tank is overstocked which it seems is likely, it will only cause ongoing problems with disease, you need to rehome some of these fish, the male and female betta need to be seperated, maybe get them a new home each, if you're short on money tupperware tubs can make good temporary homes, i've seen people on here using them.
Do you have any way of getting a picture of your setup up?