Hacksaw is fine. Wrap few layers of masking tape over the end (I leave it on when done) and it acts as a guide as well. You can't reuse the butt typically - that is just my experience. Buy a new one - couple of dollars at most. I like to jam the hole in back with piece of wine cork or something just so water doesn't go up in there when I use my rod as a wading staff.
5 minute epoxy will work or 30 min epoxy (slower drying but stronger) will hold new rubber butt cap fine.
REMEMBER - you can always cut MORE off. It is a PITA to have to add length back to the rod !!!!!
Most factory rods butt ends are too long for me, but I typically never cut them down in case I end up trading/selling the rod later on it really kills the resale value of it. Most folks are a bit taller than I am as well.
One of my favorite factory rods - St Croix Premier 8.5-2pc (my go to trunk rod) the butt end is just about perfect so no worries there.
FYI - M&D's in Wareham (mikecc here in S-B) stocks most flared bottom butt caps by Fuji or PacBay. Some prefer the slim caps without the flared ends. I have also seen guys use end caps from hardware store made for chair legs.