Some answeres.
1)You sure can fill it up again with the same flavor, no matter who makes the juice.
2)Use those empty ones first. I used to vape SmokeTip, and could re use the empty carto's till the paper fell off.
3)if you fill them by putting the juice in the clear cap and pushing the carto in you sure can vape immediatly. be sure to wipe off any excess juice from the bottom or you risk killing any automatic type batteries.
4)If you want to mix flavors in a carto, mix them together first, then put the juice in. You can also put a different flavor in a carto, but I suggest a stonger flavor than originally used. I used to refill vanilla cartos with blacklicorice juice. Never knew the difference.
5)You can clean cartos, but for as cheap as they are and seeing as you have a lot of empties, I'd toss em when they start to get hard to vape. (the juice eventualy gunks up the heating element). If you realy need to pinch penies, There are a lot of ways to clean them, but to be honnest, they won't last much longer in my personal expierence.
Good luck. And great questions by the way.