even if your ride is 126, you can generally squeeze a 130 hub in there with no problem. ...although IIRC, some ppl say that if its an aluminum frame, that might not be so good??
I think your current 6speed is a freewheel and not a cassette. I guess this doesn't matter since you are getting a new set of wheels. You'll probably need a new chain to accomodate the 8 cog spacing.
You can pretty much use any shimano derailleur as long as it isn't pre1997 duraace?? I have a first generation 105 with a 9speed setup and there is no problem at all.