IMO the words "stainless" and "forgeable" do not belong in the same sentence. I spent two years of my career trying to/forging stainless steels, and the only real thing I accomplished was ruining a lot of otherwise good stainless steel. Yes, there have been people who forged stainless steels, but honestly, it was more of a gimmick then anything. Nothing wrong with a Bladesmith making a stainless fillet knife via stock removal. You'll have far less frustration, and a far better end product.
Personally, from a Bladesmithing aspect, when I produce a fillet blade, it's usually 15N20, and it's finished out with clear Gun-Kote for protection.