Just my $.02 but I'd agree with Dave. I used the spiral duct and the long radious Y's and 90*'s. I bought the stuff from Oneida. It's expensive and there's also a cost for shipping but I think it's ideal for dust collection and very good about being re-usable or making changes to the system. If I had to do it again I think I would try to buy it locally and save some. I would think the HVAC Supply houses would carry it. At least the spiral duct.
Careful about reducing the size of the main. Reducing the size of the main slows down the air flow.
Just my thoughts.
PHM