Rescued three year old Ball and need Help

[reddsurion]

I took a Ball python in becuase the owner decided he didn't want it anymore. Once I saw the environment the snake was in I could not refuse it. The snake cage had not been cleaned for months and the water dish had green slim in it and no temperature controls for it when the house seemed about 70 degrees. The snake and environment looked so bad I wanted to slap the guy.
Took the snake to the only so called experts in town and other than some skin issues that needs clearing up the snake is fine.
Bathed the snake and put it in a clean and temperature controlled cage and in only the two days I have owned it the snake has been looking much better and is more active.
The previous owner said he had always feed live since he had gotten the snake.
The snake will not eat yet but I figure thats from the stress it been thruogh with being handled and bath and the new environment. I have read were that Balls go for months without eating when stressed. I have tried to feed it but tried with F/T vice live.
Oh yeah just FYI this is not my first snake only my first Ball Python
My question are as follows:
1. Is there any chance in changing it from Live to F/T? if so whats the best way to go about it?
2. Will he ever get to a point he is not so skiddish? He jump everytime you touch him but never has bitten. I have been handling him almost every night for a short period of time(10-15minutes)
3. He is currently in a 20gal long and I was thinking of building him a 2'X2'X4' cage. Is this size cage big enough for him?
4. At three years old he looks small. right now he is about 2 1/2' long How big should he be at three years old?

With the questions that I have asked you can see the "expert" was not much help

Thanks in advance