Is it a 20 high or a 20 long?
Assuming it's a 20 high, then:
The one DG is fine;
Cherry Barbs should be in groups of at least 5, so you could get 5-8 depending on what else you put in;
Black Neon tetra is fine too, a group of at least 6;
Swordtails are ok, they don't need to be in specific group numbers, although they have a large bioload so if you wanted the DG and Cherry Barbs you could only add 2 Swords and you'd be fully stocked;
I believe the Hi Fin Variatus is a High Fin Platy, so those would be OK too, again they don't need a specific number;
The Columbian Tetras are fine too but they need a group of at least 5;
So you fish choices are fine but you couldn't have all of them of course. So for example you could have:
1 DG, 6-8 Cherry Barbs and 6-8 Columbian Tetra OR
1 DG, 6-8 Cherry Barbs and 2 Swordtails OR
1 DG, 6-8 Cherry Barbs and 4 High Fin Platy OR
1 DG, 6-8 Cherry Barbs and 6 Columbian Tetra
What filter do you have on the tank?
I'm not sure of the swimming levels of these fish though, it might be fairly crowded mid-top level with those choices and the bottom might be fairly empty, but maybe someone else can confirm this for sure.
