There really are only 3 options...return all the fish and go fishless, cycle with the current fish (though I would return as many as you can), or using established media. The latter is the easiest way if you can get your hands on some, especially since you already have fish.
You could just give the black neons and barb to the LFS. You will be lucky to get any store credit, but that's usually the most that you will get for the fish.