I currently have a 15 gallon tank with 4 tiger barbs, 1 balloon Molly, and 1 platy. I forgot the length of the tank but I'll type it up later and all I know is that it's a long 15 gallon tank. What other fish could I put in there with my other fish???

All the others were either stressed out or nipped in the end
Most likely the Tiger Barbs will at some time start nipping. None of us wants to suggest other tankmates because of that fact. We can't even suggest other types of fast moving fish, due to the other slower tankmates and size of the tank. Not trying to be mean or anything, just stating the facts. It's a bummer that all fish don't get along, or sometimes do for a little while, then their true colors come out when you least expect it. It is possible that your Tiger Barbs will remain calm and not nip, but that would be the exception, not the norm.
I've heard kribs can work, but your tank isn't large enough for a krib.