The main issue is total number of Corydoras. Each species lives in large groups numbering dozens if not hundreds of their species, and in many (but not all) of their habitats there is just one species in an area. But they all get along, and numbers does seem to be the over-riding factor, whether it be one species or two or more species. Nuymbers are very important; the more there are, the less skittish they will be, and that means healthier.
I try to get a few of a species when I acquire them, usually five. For various reasons, this may not always be possible. For well over a decade now I have maintained a group of 40, 50 or 60 cories in the tank, and there has always been several species within this number. At present I have 41 representing 12 species; some have seven, six, or five of their own, some one, two or three, or four. Some species do seem to like their own more than others, Corydoras panda is one that always seems to be better with a group of their own, five, six or seven for example.
Not all shoaling fish species are like this; most do need a group of their own species, not mixed species. But Corydoras so far as we can tell do not seem bothered, so long as there are lots of them. Which brings me to your tank...you want more than just six. I would say 12-15 in total should be minimum, and since you like them you could go up to 30-40 and they would be even happier. If you can get 3-5 of each species, good.