Listen to me...
I have the same kind of tires as you and I also have a CO2 inflator. The thing is this...16g cartriges CANNOT be bought in bulk. On the other hand, 12g cartriges can. Just go to WalMart and go to the BB gun aisle. Get a 25 pack of BB gun CO2 cartriges. This will cost you about 12 bucks. As for 16g cartriges, they run 3 bucks apiece. If you are concerned about if the 12g will fully inflate it up to 120psi, don't worry. All you have to do is carry an extra cartridge. At 50 cents apiece, it's quite a bargain. Good luck.
EDIT: Overinflating your tires will not cause much damage. There is no big difference between say 120psi and 150. The higher psi, the less chance of getting a flat because surface area of the tire hitting the road is reduced. There are no benefits over 120psi though. Just make sure the pressure is around 120psi. Hope this makes any sense at all.
Also, to efficiently pack your cartridges, place 1 in the pump itself, and then place the other ones somewhere in your pack. Once again, good luck.