I have milled titanium, what I've found works best is to use carbide cutters, or if using hss, don't let the cutter rub as the stuff I have work hardens in the blink of an eye. I use light cuts, but that may be down to my mill being a little more flexible than I'd like.
The smallest size of cutter I've used is a 2mm carbide, and I have also used a carbide V-point for engraving using extremely shallow cuts, with a little bath of thin cutting oil.
Tim