I think CSM would make the Fox as heavy as a 21 if you wanted it that way. Making them heavier is easy for them, lighter can be challenging.
On the gauge choices, I think you're well armed for quail, wild or penraised, with a 28ga. I also find I'm confident I'm well armed for wild S.Dak roosters with my 20g guns using a 1 oz. modern hi-vel load. The .410 is not for everyone, but I've found I've been very pleased with it using 1/2 oz on wild valley quail & dove, and 3" 11/16 oz on pen raised phez.
