Well, we'd hope this would be an exercise that most of our beginners would learn to do pretty early on. The homework involved is reading here and other sites the individual fish details, looking for some consensus from different sites about the final maximum size of the adult fish. Using that, you add up the inches (fish body, fins don't count, it should be understood) in your list and see how that compares to 1 inch per US gallon, which is the point you want to be at or under for your first year of fishkeeping, to better ensure a positive experience.
So to use your case as an example and give my own thinking out loud, my initial thoughts would be:
DGs - 2.5" x 2 = 5" (note though that 2 females may be needed to one male eventually)
neons - 1" x 2 = 5" (note though that a shoal of 5 is a bit too small for the shoal, 6,8,10 better)
single KL 2,3 or 4" but I'd say 3" of fish body since they are skinny (they get 4" long, so... I don't know)
(note though that they should be kept in minimum grouping of 3 Koulis)
So my thought at this point would be "I've used up 13 inches out of my 20 inches, I've got 7 inches to acquire but I've not met various stocking minimums with all 3 of the species I've already got!"
Hope that helps!
~~waterdrop~~