I am using Rampant, and I can't mount anything except Rampant equipment in the Mk.3 Power Armor equipment grid.
Here's a list of mods I'm using, although I don't think it should make a difference. I literally can't mount items from vanilla Factorio in the grid at all.
This is the problem of some mod in your list, not Rampant Arsenal issue.
Some mod changes equipment grid categories for vanilla equipment, but doesn't include RA's Armor MK3 in the same list. Unfortunately Factorio doesn't show it in equipment/armor tooltips, because grid categories are not listed to player even with advanced tooltips turned :-(
