5/16" stainless for most tubing, .035 wall. You can get away with .028" in 4130, but the bends might not be as nice. I usually get 1/8" or 3/32" for tabs. You can buy tabs from a number of suppliers.

Rear rack might be better with 3/8" same thicknesses. I didn't really like 1/4" for anything, but I'll probably start using it for randonneuring front racks.