Clown fish lady ... unfortunately, you were lied to at the fish store. 99% of the people at fish stores do not know the basics of fish keeping such as Cycling as well how many fish (and what sizes of fish) one can put into a tank of this and that size. Your tank is indeed very overstocked right now, and I'd advise you
not to get anymore fish. A Plecostomus needs at least a
75 gallon tank, so this fish is very out of question.
For the current list of fish that you have, you need at least a 15 gallon tank in my opinion. If you want to add another platy to it, a 20 gallon tank would be better. As I've said before, a Pleco is out of question for even a 20 gallon tank, not to mention a 5 gallon tank.
I am sorry to be saying all of this to you, but it's the truth. If you haven't already done so, please learn about the nitrogen cycle:
http://www.fishlore.com/NitrogenCycle.htm as well as read the rest of the beginner's articles:
http://www.fishlore.com/Beginners.htm. All of this is for your own good as well as for the good of your fish.