I second the pipe cutter, use rubber grip gloves to hold the copper steady, of you use pliers or vise grips or whatever along those lines you'll bend the pipe and it will be a woo-hoo to work with. Slow and easy on the cutter, just finger tighten it to snug, spin it around the pipe, snug it a little more, like a quarter turn most, and spin it around again. Go till it cleanly chops it off.
As far as muffler paste, sounds like a glue? If it's a glue do yourself a favor and take it back to the store, it's glued together you will probably never separate the parts again which makes tuning impossible (the point of the kit is adjusting lengths for tuning the powerband)
If it's just for sealing up stuff like small openings, then send me a link, I need a way to cleanly plug up small connection points in my custom pipes... Been trying to do this without it looking like total crap.