That probably is a bit too much fora 20L I have 8 fish in 30L (each fish max 1 inch) and some shrimps and snails. Guppies produce quite a ;lot of waste as well! I would personally pick one or other, either the 6-8 guppies or the 4 rocket killies, and some shrimps. As you state, the guppies will have babies so that adds to the bioload etc as well? Would the killie eat the baby guppies? As you have already said, it is probably a case of pick one or other??
If you really want to mix then I MAY try a pair of killies (though you say they are schooling fish) with 2 pairs of guppys (check they are compatible first in a small tank though) at the most (plus shrimps). The other thing to bear in mind is that if it is a 20L tank, it probably only has 16L of actual water with other bits in the tank (substrate, heater, filter, ornaments, plants etc etc) and it not filled to the rim - my 25L has 20L of water, my 35L has 28L of water?
As for snails, it does add another interest to the tank, I keep assassins in all my tanks, they do breed (slowly) but even I can easily catch them when their are too many. They will help keep the tanks cleaner (glass etc) and devour anything that may die as soon as it dies, so I always include some snails!
You should also plan to do maintenance and water change once a week of about 20% if sensibly (not over) stocked.
If you decide in the future to get nano fish like the chilli/spotted rasbora then 8 of those would be what I would (and had planned for my 25L if the betta didn't survive) with no other fish of coarse?
Hope this gives you something to mull over, but I would not class myself as an expert..............
Simon