hi, I’m sing servos that have 20kg-cm. That should be ok?
I'd step it up to to 25kg-cm at a minimum, but 30kg-cm would be better. The issue with a fully flying stab isn't the power required to move it, but instead the power required to hold it still and resist flutter. You could use 20kg-cm on the ailerons since they're quite small on the F-18. The flaps need 25+kg-cm as well since they're quite large on the F-18.