4 otos would not overstock the tank, BUT they are pretty delicate fish and do much better in planted tanks. Personally, I wouldn't keep them in less than 15 gallons.
As for cories, I would not get the larger types (bronze, peppered etc) as they do need rather more room than others. I would go for pygmy or panda cories - they are smaller and can be very happy in a 10 gallon. Their smaller size also allows you to have a proper school of 6. :) Make sure you have a sand substrate, though, not gravel.