Hi Matt,
I've used Vlocity Junior and fullsize, Egg 3 and a couple of 200 shot gravity feeds. One of the gravity loaders was a bit loose, but some pvc tape sorted it. Anything bigger than a velocity (like my un-sanded halo) wouldn't fit, so you may have trouble with some hoppers. Don't think swapping to a clamping feed neck is an option. The STP neck is an unusual design to allow for changing to a stock-class feed tube.
As for barrels...
The Un1tech won't go on - the rear sections too big and fouls the pump. However I do have these shown in photo. Top to bottom - 1 Piece Dye 14" boomstick with stainless back, 14" Empire 2 piece with .687 & .684 back and the 13" CP one piece .689". I can bring them on the 18th to try out.