I'm new to the shotgun build and I have Shotgun Surgeon and just got And Stay Back. I have the Hunting Shotgun fully modded right now (waiting for a Riot Shotgun) I have fast shot, and commando and sniper for when its close enough for VATS. But I am curious to what is the most effective ammo to use against tough enemies? I've been reading that the Slugs are better against higher DT enemies, but i'm not to sure. But with the one projectile, i don't think And Stay Back would be any benefit? Should I be using Magnum shells instead?

Magnum rounds give bonus DT penetration and damage IIRC. It's what I usually use if you pick up the hand loader perk. If you have the GRA you have flechette rounds, that add a good DT penetration.