Hey there, Welcome. Personally i would not get a 100 gallon tank, im thinking that is Way to big, nor would i put them together. A 10 gallon tank should be plenty for the new guy, and keep the one you have also in a ten gallon until his size increases, then move on to slightly larger tank/tub. From what i have learned they like a closer more secure area rather than a larger area, plus it would be a little more difficult to keep temp/humidity up in that big of a tank. Have you thought about building or buying a rack? This would be a great investment for having a numerous BP's.
As for quarantining i would Deff do it, no matter what. For information check out this page http://ball-pythons.net/forums/showthread.php?t=74329