Are these screws adequate for securing a 304 stainless steel shelf bracket?
Typically, 1-1/2" (about 40mm) is plenty in a wooden stud (properly piloted). It's rare that I'd go much deeper since you increase the risk of damaging electrical cables, which are centered at about 1-3/4" (45mm). Keep in mind that the holes punched in such brackets give you a clue as to their intended load range. If they were super heavy duty brackets they'd have larger fastener holes. 

Three such fasteners will hold a heck of a lot of weight. (For extreme loads, the long side goes on the wall.) Penetrating deeper may increase pullout strength, but if you're loading the shelves that heavily, pullout isn't your primary concern. Crushed plaster and fastener bending will occur first. 

I don't know what you'd use into the shelf since I don't know what your shelves are.