I too am in the process of this with a few of mine. I have found the best way to go about it, at least for my 4, is to start with live equal sized rats sented with cage litter from my mice colony. 3 out of my 4 balls are affraid of anything that is bigger then the mice right now, so I have been using lg crawler rats and sm wen rats that i put in a 6 qt tub filled with mice bedding( the funkier the better). I just let them run around in the bedding while i thaw the other rats for my other balls and presto my 3 girls take them down as soon as the hit the tub. My one 2200gr male takes f/t sented rats of any size as long as they get roled around in the same mice bedding. He must just like the smell of mice better. He actually has never eatten live in my home and only ever had one f/t mouse, over a year ago, but ever since then he has had to have his rats sented before he'll touch them.

Well hope this helps.