Freezing them will kill them. The bacteria should ideally be kept in a tank (of course) but you could have them in a bucket or plastic container with an air stone, if you need to keep them for longer, otherwise you could simply wait until you'll have the tank before you change the cartridges.
Soaking the new and old cartridges in a container isn't going to do anything
You could try putting the old ones into the new filter simply, 50-50 and then the rest of the filter will be populated by beneficial bacteria in time as they'll spread. You could also try burying the filter pads in the gravel and hope for the best.
I had a little bit of "old" tank water when I set up my larger system, I also took the gravel from the smaller, cycled tank. Then I let a few of the fish into the big tank after a few days and after two weeks all the fish had moved in and the tank was cycled.
I've read about the shrimp/fish piece method as well, but I never tried it myself. You could try adding fish food to the tank daily instead of putting the raw fish/shrimp there
After a week of it being set up you could introduce some of the fish (after a 50% water change) and then monitor the ammonia, nitrite and nitrate daily and change 50% whenever needed
Remember to add beneficial bacteria, like Stresszyme, Nitrivec, fluid bio filter media or similar products every time you change the water! (I added a full dose of fluid bio filter media with every water change rather than just dosing the new water.)