Personally, I'm with R12 on the service evac. I carry three 7 cfm pumps in my truck, and if a mojor component has to be serviced, like a condenser, or a reclaim coil, etc. All three are going on there, for as long as I can stall the store managers.
I prefer to have ball valves everywhere, and I do mean everywhere, but on most older systems, you don't have that.
There is a thread on dirty oil right now, and I mentioned cleaning the separator.
The oil separator is the one component that everyone in mfg and install ignores. There should not only be lockoff valves on either side of it, but there should, IMO, be a bypass around it for cleaning it. I've installed several now, and they are the only way to go. You just need to make sure that the bypass is CLEARLY MARKED IN A HEAT RESISTANT MANNER that it is a normally closed valve, and should NEVER be left open, except during cleaning procedures. This type of thing speeds up evacuation during service.
As far as initial startup, I am more inclined to connect ten 7 cfm pumps to various points of the system, rather than one large pump.
I've seen one large pump left on lose a belt when no one was around, and then you lose your vacuum.
If you can stay and watch it, I suppose it's ok, but I wouldn't want to babysit a large rack overnight.
Every rack that I've started has been down to and held 300 microns. I think that is sufficient. I've also never watched a pump run, though. Go home, leave those ten puppies run, come back in the morning and check it. Not many contractors want to invest in a pump that large, anyway.
3 grand for 10 pumps is a lot easier to swallow than 2 grand for one...