re: Leftover Roux

If you're asking if it stores well in the fridge, then yes, it keeps for a long time.

As for using a pre made roux, same as making gumbo using a same day made roux, heat the roux up, add your trinity and go from there. Some folks like to cook the trinity down in a tiny amount of oil/fat/lard first then add the pre-made roux to it before adding some form of liquid, either in the form of a nice rich stock, broth or water.