I think the cartridge loops on shotshell slide are fine; just like cartridge loops on a shotshell belt - assuming we are talking SASS rules. Here is what the latest version of the Shooters Manual says:
Ammunition required for reloads during the course of any stage must be carried on the shooter’s person in a bandoleer, cartridge/shotshell belt loop, pouch, holster, or pocket or be safely staged as required by stage instructions. Leather belt slide ammo loops are acceptable; however, shotgun shell slides may not be worn over shotgun loops on an ammo belt. Shotgun loops must be in a single row. Rifle and revolver ammunition may not be carried in a shotshell loop. No ammunition may be carried in the mouth, ears, nose, cleavage, or any other bodily orifice.
When I first read it, I worried about the sentence that says rifle and revolver ammo may not be carried in a shotshell loop. Upon further reflection, I do not think that means you cannot have a cartridge LOOP on a shotshell slide next to your shotshell LOOPS. It is simply a prohibition agains carrying rifle/pistol ammo in the shotshell LOOP. At least that is my reading and I see enough pros making slides and belts with both (and shooters at state, regional and national shoots wearing and using them) that I conclude they are fine. Hope so; I am working on shotshell belt for daughter that will also have cartridge loops.