Pet stores are not worthy of being trusted ever.

You have to look at it like this.
You don't want to plan ahead when buying a snake too much, you have to GROW your enclosure and hides as the snake GROWS as well. :]

A 20gal will fit him nicely as an adult, but for now, lets get him eating.

Definitely try live, if you haven't tried it it's worth a shot. As long as you do it responsibly and don't leave it in too long you don't have much to worry about.
(especially when you feed live pinkies or fuzzies from your rats because then they REALLY can't hurt the snake)