People use many different sizes of 6" PVC pipe for their ducting. I've measured interior diameters as small as 5.875 and as large as 6.175 for the various Styrene and PVC pipe. Outside diameter adds two thicknesses of pipe that can vary from as little as 1/8th to over 3/8" total.
As Ed says almost all flex hose is designed to slip over a 6" metal pipe that can be up to 6 1/8" in diameter. My approach to making all fit is fairly easy, but takes a little math to do it right. Knowing the circumference formula for a circle is Pi * D where D is the pipe diameters, I calculate that my 6 1/8" inside diameter pipe has an inside circumference of very close to 19.25". That means an ideal pipe should taper from a circumference of about 19" to about 19.5" to make a good wedge fit for our hoses.
I then throw a cloth measuring tape around the outside of my plastic pipe and measure its circumference. For the most commonly used S&D pipe this gives me very close to 19 5/8", a full 5/8" too big. Knowing that my standard saw kerf is 1/8th inch, I can cut six equally spaced kerfs in that pipe about 4" deep and be assured the result can be squeezed down to fit inside almost all flex hose. I can then use a band clamp on the end to squeeze the pipe together and a little glue to hold all in place. Actually that is not needed and a better way for me is to use some standard 12 gauge insulated wire, make a starter hole and then wind that wire in a spiral so the hose can screw right onto the pipe. A second end hole holds the wire in place along with a little superglue on the loops. This not only holds the pipe together, it lets me install the hose without any costly clamps and creates joints that don't leak. Sometimes when I cut too deep, a piece of aluminum foil HVAC tape at the upper end is needed to seal my cuts..
Another much harder technique is to make a tapered wooden mandrel with a wire thread as above that can be screwed into the plastic pipe. When the pipe is warmed with a heat gun it will easily accept that mandrel. Leave it in until it cools and the result is as a female fitting that you can screw your flex hose into also for good tight joints. This requires no cutting, but requires making a mandrel and taking the time to work over the pipes with a heat gun.
That's about all I know about connecting the hoses to the plastic ducting.
bill pentz