It depends on several variables, including your individual fish. Harlies are quiet common, I am a little surprised you can't find any. I would just keep looking, or ask stores if they can order some for you. They would probably group up with similar fish, but that seems to vary in individual circumstances. Often times, small schools in aquariums don't really school all that much anyways.
As for the barbs, they will not likely pay much attention to fish other than cherry barbs. IME, they are not tight formation swimmers generally, and they are generally very concerned with hierarchy and flirting. They are also quite common, so I am sure you can find them eventually without too much trouble if you want to add a few more.