I am a "belt and suspenders" kind of guy. I carry tire plugs as well as sticky strings and CO2 cartridges and a Slime air pump and a mountain-bike type (short and fat) hand air pump, which ties nicely to the frame tubes with a couple of Tyraps. I figure the CO2's only have limited capacity and the 12-volt pump can break at the worst time, but the hand pump is pretty fool-proof, although it will take forever to get a rear tire filled.
As for the CO2 cartridges, the ones for pellet guns have a smooth neck and rely on being clamped up against the puncture fitting and held in place by the clamp. The ones for the tire inflator kits are threaded on the end so they can screw into the tire valve fitting, so the two aren't interchangeable. The threaded type come indifferent sizes so if you want to use them, get the biggest you can that will fit your fill valve.