"In Vietnam, supposedly the Australians had squads or sections equipped with both FN SLRs and AR-15s (or M-16s). It sort of makes nonsense of any problems that may have been incurred with the ammunition supply but all through WWII, the British, the Americans and the Germans all fielded low level units (squads, that is) equipped with weapons using two different cartridges. I couldn't possibly say if it made a difference but it was done."
Actually, no, it doesn't.
Having two, three, or even four separate and distinct cartridges (rifle, machine gun, handgun) isn't that great a difficulty as long as you have that composition throughout the entire force structure in the field.
You ship everyone a mix of everything, and generally your people will be sufficiently armed.
It's when you get into a situation like what they faced during the Civil War that things start to get REALLY screwy.
In one cavalry division you might have troops armed with Morse, Sharps, Peabody, Gallagher, Burnsides, and Smith carbines, each of which requires a different cartridge.
In an infantry division you might have troops armed with .58 cal. Springfield rifled muskets, .69 cal. smoothbores, foreign contract purchase rifled muskets, Spencer carbines, and few others thrown in for good measure.
The fact that small arms supply even worked at all for the Union during the Civil War is a frigging miracle, but every new gun thrown into the mix made the situation even worse.