First off, unles you plan to upgrade to a much larger tank, get rid of the common pleco. He will be way too big for your tank in a very short perios of time. With him in the tank now, you are definitely overstocked as they get big and are also big waste producers. If you get rid of the pleco, you could probably add more fish but you won't be able to add many. Based on the 1" per gallon rule, you are probably getting close just with the neons and platies.. Just be careful putting a betta in with guppies (I see you said small tailed) or swordtails. A male betta will view anything with a large tail as a rival and fight it