Annie,
For fillets that are structural elements, I use a mix of wood flour, silica, milled glass fiber, and sometimes, depending upon how strong I need it, chopped fiberglass.
For bulkhead to hull tabbing, I don't use the heavy duty mix. It's phenolic microballoons to keep it light and some silica to make it thixotropic. I go by the school of thought that the fillets themselves are not structural components. Some reputable builders are using merely foam to form them to avoid hard spots, especially on the thinner layup hulls that are de rigueur these days. Not that current building standards are necessarily all that great, of course. The Pearson 10M is a hefty layup, though, and in any case, the structural strength comes from the well-radiused glass tabbing. I know that you know all this - just clarifying my technique.
I should add that, perhaps overkill, the edges of the plywood bulkhead, even though they are getting buried in the filleting mush, all get three coats of epoxy to seal them before going into place. I never, never, never want to replace one of these babies because of rot.
Incidentally, the original boat had about a 1/2" gap between the bulkheads and the hull, kept off with a few tiny squares of what looked like carpet remnants, no filleting material at all. The glass was just laid on, bridging across the corner to make its natural radius. Water eventually worked down that channel from the above (or condensation, perhaps), rotted those little carpet pieces, and caused the nastiest boat funk smell. I even found an ant colony in one of them. They hadn't worked their way into the wood, which was surprising.