So long as they're all Colisa lalia Dwarves they're the same species, and thus the different colors can breed.
Some fish labeled Flame Gouramis are sometimes Trichogaster chuna though, so double check with Google Images.
It's about a medium difficulty for mating, some do it without much interference, others need a large variety of factors to get in the mood - it depends on the individual fish. But if all your fish are brightly colored you have all males. Female true Dwarfs are generally silver to grey to pale in color.
A severe lack of trichogaster.
Just because your Gourami is sick does not mean it is always Iridovirus, DGIV, Gourami Disease, et cetera.
Look at all the other factors in your tank before coming to this conclusion.