I have a few questions about re-using yeast. 1) Is this worth doing? I was told it adds character to the next batch. 2) When adding old yeast to a new batch, do you also add more fresh yeast? 3) If so, how much? 4) How many times can this be done? I was told a local microbrewery does this 7 times before dumping. 5) If kept in the frig, how long will it keep? 6) Can different old yeasts be combined, or must they be kept separate? Any answers or suggestions would be appreciated.