So between the Cory’s and rad peas I can add 6 Cory’s and 9 rasboras so add 15 fish? Can 20g accommodate this? Is there another gourami I can get? A female as a tank mate for the male dwarf cobalt?Definitely increase the cories. In a 20g, I would aim for 9-10 miniimum. Same species of mixed, but if the latter try to get say 3 of each species. This increase will make a big difference in their health and well-being.
As for other fish, the tank size is limiting. With the gourami, you want peaceful shoaling fish, no species even remotely prone to fin nip. Rasboras are good for this, the smaller species, such as Trigonostigma espei or T. helgeli, a group of 9-11. These swim mid-water, so you have a nice mix of substrate fish, mid-level fish, and the gourami cruising upper/mid level.
So between the Cory’s and rad peas I can add 6 Cory’s and 9 rasboras so add 15 fish? Can 20g accommodate this? Is there another gourami I can get? A female as a tank mate for the male dwarf cobalt?
How about five Trigonostigma espei and five neon tetras?Yes on the numbers in this tank.
No on another gourami. This is more a risk than a stocking problem. The gourami you have seems settled, don't rock the boat. All male gourami are territorial, some more than others. He might well decide he has his space, intruders are not welcome.
That's not enough of either. Schooling fish need to be in groups of at least 10.How about five Trigonostigma espei and five neon tetras?