Is this overstocking my tank?

the most you could add without having problems in the long run would b the 4 danios, the 3 cories, a honey gourami and one more fish. I wouldn't get just one platy because they like being in at least pairs. Having all the fish you want in that tank will over load your system and you will have access nitrates that don't need to be there. But I wouldn't go any more than that. A lot of people tell you about the "one inch per gallon" personally, if you go with that you will have water quality problems and your fish will have no room, it should actually be 1 inch per 3 gallons for a healthy system. Take my work for it I have done freshwater for YEARS and I am now onto saltwater, so I know what I'm talking about.