1/2" shanks for as much as possible, 1/4" shanks for trim routers. Collets are cheap compared to quality router bits, so if you find an 8mm shank bit that you just have to have, go buy the collet. Those three sizes are the only ones I have, and I've never wanted/needed any of the other sizes. To be honest, I couldn't tell you right now if I even have an 8mm shank bit. At any rate, when you select a router, make sure that you know what size(s) of collet it comes with, and what you can get as an accessory.
I've never been tempted to buy a collet reducer, as I've always been worried about the tolerances/quality, but they are available if you need to reduce from, say, 1/2" to 8mm. Anybody out there have experience with collet reducers one way or the other?