In this situation you have a very passing Betta, just the reason it can be kept with females is astonishing, If it can be kept with females then it can be kept with gouramis too mate. Flame red's are very bright and I have seen bettas show aggresion towards them. If the OP's Betta has been flaring at the DG, then it is not going to go well in the end mate.
PS, not every situation is the same, with yours it's different. Your females have quite a bit of color BTW, so it's not a big suprise at all that it's fine with the DG