I'll agree with Alshain.
Add .....if anything.....two ottos to help the school.Neons are low biolaod/fish but they still are fish and will contribute.
They will get to 1.5 inches.
The platies may not get to four or even three,but you can bet they want too...a little bigger then what you think is where they would like to be...
On a side note,it's a small tank and being so you need to work on keeping the biolad low imo.With small water volume changes in conditions happen fast and it hardly ever changes fast in a good way.(read this last nugget twice,I had to and I wrote it lol.)
just my opinion as always