DMNemec

Here's the only troopers with the ammo pouches. Both have a vest and pauldron, too. There's a 3rd trooper with a vest and black pauldron, but he's not facing the camera so you can't tell if he's wearing the ammo pouches.

For reference, here's all 3 examples I could find showing FO HWTs wearing the rank pauldron. Of note is that the two you can see the front of both have the MP40 pouch. It would appear that anytime a HWT has a pauldron on, they also have the MP40 pouch. The only questionable one is the guy with his back turned. It also seems like White, Black, and Red/Orange are acceptable for them. Not 100% sure on black though, the color was a little hard to make out on that video. Edit: Turns out that 3rd one is black. I watched it in HD on the big screen today to figure it out. Here's the gallery.

There's also a Stormtrooper behind Poe on Jakku with a sling on his blaster, too. Better closeup of the attachment method.