HELP!! fish dramas.Think the petstore guy was not in the mood for helping today!?

We bought a 21L tank a few days ago. Set everything up and left it like the initial storeperson suggested. Took a water sample in 2day to test at the same store and was told it was fine and we were ready to choose fish. My kids were with us and my son chose neon tetra(3), we also got 1 silver molly, 1 balloon molly and a gold platy. I told the guy we had a 21l tank and if that was too many for it. He said should be fine :/ jst reading up some more and seems i have 2 many fish! also the balloon molly is nipping the gold platy ( iread it was normal for them) but the platy now seems stressed! also one of the tetra seems to be on its side (not sure if its resting) Im so angry because i know i asked the right questions about aggressiveness, size, pH, food, temp etc...and now Im worried! dont want my kids to get upset about having to take some back/give away.