20 gallon stocking question

[HeatherW]

Hi, new to the fish world, but think I have a handle on how everything works. we will see

I have a new 20 gallon tank and want to know if I do the following stocking plan will I be overstocked.
7 cherry barbs
2 platy
1 dwarf gourami

want to add either:
1 molly or balloon molly

or can I switch the molly out for 2 more platy??