Yes many species will remain "together" depending upon the species but I am worried about the reason for the question. Rasboras are shoaling fish, this is in their genetic makeup and they expect a largish group. Ten or more has been demonstrated as being essential; the negative effects of fewer are rather startling. So if the question was aimed at getting for example 4 or 5 of one species, the same of another, and intending them to be "OK" the answer is no. A group of ten of each species might still swim around together much of the time, that's fine; but the number of the individual species is clearly very important to the individual fish, if they are to lead healthy normal lives.
I've never kept Harlequins in with the smaller rasboras, so that might be different. I have had Trigonostigma hengeli (one of the smaller "Harlequin" species) in with Brevibora dorsicelata and they were always together, but the latter species had been with me several years and there were four or five left and they were undoubtedly feeling a bit lonely alone. This situation often comes about when a group of fish begins dying of old age and very few are left; I think it is kinder to them to leave them in their familiar surroundings, provided they do not react with aggression toward other fish as a result of their low number--which can unfortunately sometimes happen.
