Geth Pulse Shotgun is great on support characters who use shotguns. The best of all the shotguns for them, in Mass Effect 2.
For squadmates, the Vindicator for assault rifles.
The Mattock is less useful in the squad's hands than it is in Shepard's. Squadmates with the Mattock fire much slower. Its accuracy is much better than the Revenant, and it hits hard while still being able to fire as fast as you can click the mouse button. In the hands of a Shepard with AR training who is not a soldier, it is probably the best of the AR weapons. In my opinion.
These opinions change in Mass Effect 3, because the squad AI changes in Mass Effect 3 and the weapons stats are different. In ME 3, I equip the squad either with the Avenger, the Geth Pulse Rifle, the Prothean Particle Rifle or the turian AR, because the squad is accurate with any weapon and the bursts they fire are based entirely on clip size. So I throw in extended clips and the piercing mod for them.
As for shotguns in ME 3? Balance damage with rate of fire, slap on the heaviest damage barrel and piercing mod you can get, and that's what everyone other than Shepard uses. Weight doesn't matter for the squad. Accuracy is always very high. Just do not use anything that requires charging shots for the most damage; the squad AI in ME 3 doesn't know how to charge a shot.