to cut it to length I use a kett shear...depending on diameter should be able to cut 99% of it, then use sawzall to cut seam. Cutting in registers I square from floor snap a chalk line on top and have a cardboard pattern, trace, use a 3/4" stepdown bit to start and use snips to cut them out, bulldog snips to cut the seams....don't do enough to justify a plasma cutter but a sawzall will work also.
If you are going to paint it get the stuff that you can paint,I forget what it is called ask supply house.
Its called Iron Grip and apply generoulsly it will shrink, depending on size of spiral dont cut the seams..the duct is under tension and the top and bottom of the cut will blow out, leave the seams and cut out your register taps, sprial duct is something not to be attempted if never done before...Ive ran litteraly miles of this style ductwork...hanging it to make it look professional and level is a experience in itself, many different ways i.e threaded rod, 16 gauge hanging strap with proper circumference bend, 1/2 belly band with companion flange and rod...I've seen some bad spiral jobs. As far as cutting follow the circumference of the pipe at your cut measurement and make hash marks with a sharpy and then follow up with a sawzall, its loud but quick..
Just did a small spiral job. Reducing trunk line. I used sawzall for the pipe lengths and unibit to start the holes for my registers then snips. I used the curved registers, not the boxes. How can you install the registers without cutting the seams??