All beautiful fish but a little over crowded.The rainbows need more swimming room than 20 gallon.
The 2 Dwarf Gouramis sound good. 4 corys and 7 neons. If you think you will need some thing for algae control then 4 or 5 Otos instead of the corys would be good. they have a smaller bioload also.
Adding the rest of the fish would definately overcrowd the tank. Although in a larger tank it could work.
Definately better without the guppies. 2 Dwarf Gouramis, 7 neons and 4 corys(the shrimp will be Gourami lunch) ? Sounds great. Let us know what you decide. And of course we need pictures. We love pics