Pound the stopper? Woah! I've never had to pound my stoppers in. I just push them on, firmly. The air lock will let the gas out just fine, or it should. Your stopper is simply to hold the air lock or blow off tube and to make a form fitting seal. But if you go to pounding it in, you just might have a heck of a time trying to get it back out!
I do know that there are different sizes for the glass carboys vs the plastic carboys. my LHBS was able to tell me what size I needed (my carboys are plastic).
There was a difference in that the plasic carboys have a bigger opening than the glass ones, and the lady that runs the place was able to sell me the right one. Which was a #10.
But in anycase, you are not trying to seal the carboy like you would a beer bottle with a capper! You simply want to make a firm seal with the air lock.