There is a lot to be considered when you stock a tank. The one inch per gallon is pretty much bunk because every tank is different and has it's own personality so to speak. With neons you can fit quite a few in there say fourty or so and see how they do. You can always add more later if you want to. Your tank will basically tell you what it can support.
my suggestion is to start with a school of 15 or so if you already have the other fish in mind and then if you want more later then add to it but i like to give my fish more than ample room to race around at full speed and make it seem like there going somewhere just more natural behavior