The whole "inch of fish per gallon" rule is absolute garbage. Too tell you the truth, the best way to tell if your biologically over stocked is to take a look at the waste each fish is producing versus your filtration and water change schedule.
For example, my 150 is technically overstocked...but factor in FX5 filtration along with 3 Emperor 400s and a 50% water change every 4 days and you can bend the rules a bit.
If your dealing with single power filtration and standard tank maintenance, then I would err on the side of caution versus test the biological limits of your tank. Plecos put a strain on the bio-load of any tank...they are poop machines. The danios are clean, less demanding fish so that helps.
I think you could add a couple more fish, but that's about it. You have to factor in that your gas exchange is not as effective in a standard 20 gallon versus a 20 long due to the restriction in surface area.
I hope this helps.
