A safe rule of thumb would be......1 POUND of fish per 5 gals.
Of course, you have to have the correct amount of growbeds to support the fish also.
You could start with less GB's , but you'd have less fish too.

Cats can survive even if there is ice on the water. I'm not sure about the others, but they are game fish too, they might be able to go pretty low.

Might find some info here....

http://aquanic.org/publicat/usda_rac/fact.htm