Late on the scene...
I find this thread very interesting, especially since I was stuck trying to figure out what to do with my fifteen gallon tank for the past year.
Amazingly I've came to a similar solution:
- 3 Platies
- 2 Ottocinclus Catfish
- 2 German Blue Rams
- 3 Marbled Hatchets
- 1 USD Catfish
It's worked out well for me, The addition of an Upside Down Catfish looks good on paper, but for me It hasn't turned out well at all. Sadly, all he does is hang out in a log ALL DAY LONG! He never comes out and swims for me, which is too bad. However, he may, once he settles in better, come out, right now he's not looking promising. The problem I'm having with this tank at the moment is the lack of a real bottom-dweller (like the cories you have planned)
The otto's are quite finicky eaters, I bought some algae waffers, and I have yet to see them take a serious interest in them. Also, give these guys the royal treatment when you introduce them into your tank, out of the seven I have bought so far, I have one left, all the others, with the exception of one, died shortly after me introducing them into my tank. The only stuff mine eat for me is the brown algae that grows on the gravel and glass, but that is all they've needed.
Hope you have fun with your twenty!