I think the creator of the M2 is who would know for sure. Most PSU will turn off very quickly if shorted, but the M2 is pretty unique. If they say it will protect itself from shorts by powering off quickly, then I wouldn't worry about a fuse for either of them. Just make sure the power wire to the M2 has a fuse close to it's source to protect the wire. Then the M2 has a fuse to protect itself.