I would use 6" PVC (drainage stuff is far cheaper) but the fittings
are confusing. *How does one get from 6" PVC to 4" & 2.5" DC ports?
It seems everything has a different "standard", including DC ports
themselves.

Yeah, that's the stuff, 6" drainage, I just had to go to an irrigation
type store that specializes in sprinklers to find it. They had wye's
elbows and reducers also.

My research took me to this DC guru's site http://www.billpentz.com/woodworking/cyclone/index.cfm

He indicated to use 6" all the way to the machine if possible. I have
done that with my TS it works great. At my shaper I used a stock home
heating sheet metal box to transition from 6' round to 3 x 9 rectangle
and built a ply box. For my edge sander and oscillating (dildo) sander
I branched with a 6 x 4 sheet metal wye, again home heating type to
some 4x pvc, then flex and I just gate both together at the 6" so both
on or off to afford full air flow. Bill's contention that reduction
just restricts flow. In one location I reduce down to a single 4" for
my wide dbl drum sander.