"It's a hobby. It's supposed to take a long time." --Unidentified member of the New England Model Engineering Society.
Can't help you much on mandrel use. I bought a few used ones, eons ago, and I've never used them. I generally make a stub mandrel, in place, as the need arises. But I'd guess:
Press on only firmly enough so the work stays in place, take light cuts.
Hole size shouldn't be affected significiantly.
If you're planning to use a collet to hold the mandrel, why not make a stub mandrel?
Try to make a living, not a killing. -- Utah Phillips
Don't believe everything you know. -- Bumper sticker
Everybody is ignorant, only on different subjects. -- Will Rogers
Law of Logical Argument - Anything is possible if you don't know what you are talking about.