I agree kasakato. They are pretty cool. But you just can't keep one in a 20g. 
So have you decided? Are you going with my suggestion? Because if you are, I have a change to my suggestion.
9 neon tetras
2 small cory's
2 otto's
3 platies
1 guppy
The reason I think you should get 1 guppy instead of 3 is because they breed like well, guppies. And yes you should get all males of platies and a male guppy. The reason you should get males of those is because I have heard platies and guppies will mix and breed therefore making a mixed species. Plus, I am sure you don't want breeding of anykind anyway.
And about the otto's; I think they will help control the algae while the cories take care of the uneaten food. The thing about otto's is I think you should put in 1 algae wafer a day to make sure all of the bottom-dwellers get enough food.
Good Luck!
~hc

So have you decided? Are you going with my suggestion? Because if you are, I have a change to my suggestion.
9 neon tetras
2 small cory's
2 otto's
3 platies
1 guppy
The reason I think you should get 1 guppy instead of 3 is because they breed like well, guppies. And yes you should get all males of platies and a male guppy. The reason you should get males of those is because I have heard platies and guppies will mix and breed therefore making a mixed species. Plus, I am sure you don't want breeding of anykind anyway.

And about the otto's; I think they will help control the algae while the cories take care of the uneaten food. The thing about otto's is I think you should put in 1 algae wafer a day to make sure all of the bottom-dwellers get enough food.
Good Luck!

~hc