I've flown single fairly large foam gliders (a shuttle) on a roughly 3 inch (Christmas wrapping paper tube) rocket powered by a D12 before... it works well but they do tend to arc a bit... main thing is don't launch in windy conditions!
I think the off-center mass tends to make them arc more than the additional drag. I never experimented with counterweights to test this assumption, though. The larger the core rocket and smaller the parasite glider, the less pronounced the arcing effect, however... and of course a good high-thrust motor that gets things up and moving quickly certainly helps (from an arcing standpoint anyway).
The main thing is to have enough motor power to actually get the thing up and moving in a stable manner-- IE without arcing too much at liftoff, and to keep the stack moving at a decent rate as drag increases (remember that drag quadruples as speed doubles (drag squares) so the faster the rocket goes, the faster drag builds up to retard it's gaining speed...) I tried this glider on a C6 powered rocket and it was arc-to-crash... bout demolished the rocket, though the glider was fine... stuck to D's after that... For this reason, I'd avoid D9's and stick to the higher average thrust motors, where you have a choice...
As for the aerodynamics and stability, well, obviously you want to mount the glider pretty far aft-- usually just ahead of the rocket's fins. This usually puts the glider far enough back that it tends to shift the CG back SOME, but since gliders tend to be very light for their size, (mass/area ratio) the CP usually shifts back more to compensate, keeping the stability the same or slightly increasing the stability margin (CG/CP relationship in calibers). If the glider is free to rotate on a forward mounting pin, then it's contribution to stability will be slight-- (look up "pivoting forward fins" for more information). What's bad is, they tend to foul up the airflow over the existing rear fins of the rocket, which reduces the corrective force the existing rocket fins can provide, because of turbulence generated by the glider's wake during boost. Fortunately most rockets seem to be able to handle this "bad air" without too much tail-wagging or other bad flight effects... (at least in my experience). For this reason, I tended to prefer a secondary attach point near the rear of the glider-- I experimented with a tape loop (tape folded back on itself and attached end-to-end) to "pin" the tail of the glider to the rocket tube at the rear... it worked okay, but the connection isn't particularly rigid... didn't foul up seperation, but then this glider was pretty substantial mass-wise and the inertial and simple "peg" style hook into a hole in the side of the body tube came loose easy when the glider seperated-- which then caused the glider to pitch up sharply and rip it free from the tape (or the tape free of the tube). A tape loop on the back of the glider certainly doesn't help with a clean glide, though. Later I notched the back of the glider a little bit to slide over the leading edge of one fin a little bit, right at the root/tube joint, to give a little more "stable" connection with less "play" in it from the dinky tape loop. If the rear of the glider is pretty well fixed with respect to the rocket, then the gliders' wings/rudder will contribute to the rocket's stability far more than a simple "forward peg mount" will that is free to rotate and flop around some...
In the end, it all comes down to experimentation to find out what works, more or less...
Good luck with your projects! OL JR