Any one of the three barbs, fine, and in those numbers, also good.
The three barbs are upper fish. The cories are substrate fish, basically. Having the intended group of barbs, along with 15-20 cories, is not overstocking because the two groups of fish are occupying different levels. Numbers of individuals of the species (or in the case of cories, one or more species together) is the crucial factor, more than total numbers in the tank. If the numbers of each species is not sufficient to meet the fish's expectations, the ramifications on the biological system is greater because it situation is detrimental. Just taking numbers out of the air, 12 Odessa Barbs and 12 Melon Barbs is overstocking, considerably. But 12 Melon Barbs and 20 cories is no where near overstocking.