"AES/newspost" wrote in message 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

Controllers will preface the number with the appropiate scale. Additionally,
Mach will be a 2 digit number whereas Airspeed will be a 3 digit number.

Do different levels of ATC consistently use only one of these units and
not the other?

No

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)?

Both units are on the same instrument simultaneously.

What's the smallest _civilian_ aircraft that will
have a mach meter?

The 6 passenger CitationJet comes to mind.

Do prop or turboprop airliners have a mach indicator?

Most do not. Props are not efficient at airspeeds of Mach concerns.

D.