Listening to audio channel 9 on United, at higher altitudes and during
cruise I hear ATC say things like "What's your mach?" and "Maintain
mach 77" and at lower altitudes and during approaches "slow to 180" or
"maintain 250 for spacing".

I appreciate that these are occurring in quite different ranges of
operation and the units involved are very different in scale, but I'm
still curious about the potential confusion, or at least possibility for
same, in using two different sets of units for the same quantity
(especially when you recall incidents where confusion over units has led
to loss of satellites, or airliners running out of gas).

Do different levels of ATC consistently use only one of these units and
not the other? Do cockpit instruments read in both units, or are there
two separate "speedometers" (or a switch that has to be thrown to read
in one or the other)? What's the smallest _civilian_ aircraft that will
have a mach meter? Do prop or turboprop airliners have a mach indicator?