Sometime barbs and tetras nip other fish fins. Not all of the time though. For example I had some big Serape Tetra with neons, which ate the end of the neon tetras. Now I have some small barbs and they don't bother any of my fish, even 1s with long fins. Neither would bother the pleco. The guppies probably wouldn't be good with barbs or tetra. I think is best to get them when they are small, young.
At
aquahobby someone mentioned that his barbs got aggressive when he had 9. Having a school of 5 is recommended.
The general rule to how many fish is to 1in per gallon. More can be added if you have plants or a very strong filter.
Have you cycled the tank? If not should check out mean to do so. There is fishless, and with a few fish like danios that are very hardy.