There are no Stupid Questions...
I would not recommend interchanging bushings OR tubes for any pen kits. If you buy a PSI kit, get the bushings for it. Same with the others. The Berea cigar pens, for example, will not work correctly if used with CS tubes or PSI tubes. We have blown up so many kits this way! Make sure you buy the correct replacement tubes too.
It will cost you a few bucks up front, but not as much as one blown pen kit will cost you. You can turn 10 or more pens from one set of bushings, so think of the bushings as costing 30 cents per use (3.00 divided by 10 pens), and not thinking of them as 3.00 per set.
Just my two cents' worth, for what it's worth!