Keita I don't think you will screw up your atty by using those pre-filled carts. On the other hand, there are probably very few who actually like the flavor of those factory prefills.
Get yourself some e-juice samples. Pull the poly fill out of the pre filled carts and rinse them out under the faucet. Pat them dry with a paper towel. When they are good and dry, put them back in the cart and drop in your own juice.
They can be reused/refilled several times before they wear out. That is, they will start tasting odd, or form a caramalized layer on top that prevents liquid absorbing in or out.
Eventually you will have to either replace the filling or order empty carts.
Currently I am using the blue aquarium foam as filler. Works pretty well once you figure out the proper density to stuff your cart with. Varies depending on the thickness of the juice. Not just any blue aquarium foam. Marineland brand, Rite-size U. My Walmart doesn't carry it, but Petsmart does.
Also, there is the polyfil at Wal-mart that works pretty well. You could fill about 10,000 carts with one bag of that. Get the crib size. Its layered to about the right thickness.