Get two more tetras they need to be in schools of 6 or more, preferably 8 but i don't think you have the space for that many. If under 6 they will get very stressed and neon tetras are delicate fish. I don't think you should get the loaches because you already have a lot of fish. I would never overstock a tank because maintenance will become very difficult for you. Instead I think you should make your tetra school a little bigger and fix the male to female ratio of the platies. Bring back a male and get a female or they'll start being a problem.
Is it a 20 long or a 20 high?