I'm not sure where you're getting 12" from. If the footing is 16" wide and the wall is 8" thick (guessing), since the wall is centered on the footing that leaves 4" of footing, which your slab is resting on. If I read correctly, you plan on resting your post on the 4" ledge of the footing, which has the potential of cracking said footing.
This, my friend, is why everyone says to get an engineer. If you plan on proceeding without expert on-site advice I'd suggest you do one of two things. Either rest the ends of the beam on the bearing wall and foundtation wall or, better yet, dig out and pour a separate 16" x 16" x 12" deep footing for each post and rest it on that.