Disregard the lower impedance setting described in the manual. Always leave it set at 8ohms regardless of the impedance rating of the speakers used. The lower setting is simply a safety precaution which makes overheating less likely. It does this by lowering the maximum voltage which the amplifier can supply, and because of Ohm's Law, when the available voltage is lower, so is the current and power lower. So, although the risk of overheating and fire is reduced, so is the maximum performance capability.
Yes, there's no problem with using speakers that have different impedance ratings. The M60s should be fine in a room of that size, but if you really want the M80s you shouldn't hesitate because of the 6190. It isn't possible to be "sure" that an amplifier, regardless of whether it's in a receiver or separate, will handle a particular set of speakers, source materials, loudness levels used, etc., until it's actually tried, but folks have been using the M80s and other 4ohm rated speakers for years with similar receivers without a problem.
Since you want to use direct radiating speakers for surrounds, it's suggested that the M2s would match your other Axioms better as back surrounds than the M3s would.
