Dont bottle in a growler. They generally cant handle the pressure and you will have a giant bottle bomb. Your best bet to save yeast is to harvest it after primary fermentations. Do a search for yeast washing and you will find some threads on how to do this.
After initial carbonation, leave the Co2 hooked up to the keg. If you have it set to the PSI for the carbonation level you want, it will maintain you beer at that PSI.
I just bought the 2 keg setup from Midwest and I love it!
Primary1: Pumpkin Ale
Primary2: Christmas Ale
Secondary: Sour Brown
Drinking: Warrior Pale Ale, Wit, American Rye