1) how you do Mach number separation? ie any rules about if 2 aircraft are stacked on top of each other, if you issue a difference of say M0.05 (so one at M0.8 and other at M0.75), it will take them X minutes to be Y Nms apart?
M.01 is about 6 knots; so .05 less should give you 30kts, rule of thumb, half a mile a minute...
Works fine on cruise, descent is effectively useless using mach numbers; can work but not very 'control affective' due to the transition to IAS; different levels for different types and companies, crews... etc.
IAS works well below FL250 generally speaking.
2) in this case would you consider using vectors to separate? or is speeed control preferred in the cruise?
It would totally depend on what you needed and how much time/airspace you had to deal with. 200 miles, slow down works great, 50 miles, almost a must vector scenario... My job is to space for Approach, i.e. arrivals, we need to put them across the fence at 12-15 miles gaps; I have 35 miles (80-45 Sydney) to achieve this, so it's vector city (when I'm not holding) in my part of the world; although some space is achieved by outers for jets, I "blend" in the turbo props...
3) any rules of thumb for separation in terms of in the approach phase, a difference of X knots between two aircraft starting together at the same point will mean that in Y minutes they will be Z Nms apart?
120 miles to touch, 250IAS vs 300IAS will give you about two minutes at the threashold... But I'm no flow.... For me it's space by vectoring and match speeds, far less to worry about; and probably more accurate.