What do you mean a pinch effect? I have a chance on getting a set for a pretty good price with the front being 120/70/17it would, but theres gonna be a slight pinch affect.
about a 1/2" too small rim on the 6...
unless its free, or VERY cheap, id go with the recommended size.
Not necessarily. Because it's on a narrower wheel its going to have a different profile than it would on a 6 inch wheel, most likely just going to be taller than the 180/55, which will have an effect on the geometry of the bike. But yes, I would just stay with the 180.It will handle differently. Slower going into a turn. Personally I would keep the 180.