Oh the Pygmy cories stay teeny and love big groups. Iíd do at least 6 in a 10. Thatís how many I have in my 10 and I wish I had more! I had more but lost nearly half in the couple days after adding them (big box store source and they werenít in great shape, unfortunately.) They were out in the open constantly until their numbers got down to 7 or 8 and then now that there are only 6 they hide a lot more.
Their bioload is minuscule. Between them and a ton of shrimp I still barely measure nitrates at all.
Cories are very social and will do much better in large groups. Just two or three in a group wouldnít be ideal. Do your research on the species your lfs
can get because even among the non-dwarf species some of them do get bigger than others and would be less suited for a 10.