[fashionfobie]

I got the impression that red-tail sharks can harass cories. I personally don't have experience with red-tail sharks, but this is something I have read more than once.

 

[fashionfobie]

This might be a useful tool for you to check out too. It is at least a nice guideline.

AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor

I would not break up the cories. They are very social and shouldn't be kept in groups that are fewer than 6.

I hope this helps!