switching to prekilled could be a little difficult, just depends on the individual snake. mine swiched with out a fuss after i left him alone for a week to get used to his new home. all you can do is try frozen and then thawed (F/T), if that doesn't work maybe buy a live and prekill it right before feeding. i am pretty sure there are tons of threads about switching form live to F/T, switching from mice to rats, ect. you can use the search function and might strike something.

starting out small will keep the stree of the snake down, but you best bet is to go with a tub set up. they're cheap, hold heat and humidity very good, and bp's thrive in them. you can start with a small tub and move up as they grow. if you are going to stick with glass tanks, i would go with the 20 gal and make sure to put alot of "stuff" in it to make the bp feel more secure. i don't believe you will need anything larger then a 20gal.

as for housing them together, it can be done but it really shouldn't. housing them together can result in canibalism, extra stress on the snake, and unwanted mating (which could kill the female). again there are many threads with this topic and things go much more indeph, time to search...

also i am sure you have, but check out the caresheet on ball pythons, and all the stickies that are in the forums, they can be very helpful.