i just had a bad experience w/ quarantining a fish for only a week, but it was from petsmart...
i actually got a 20long (20x12.5x13 LWH) last night and set it up w/ the filter from the 10g (since the 10 will be made into a sump for my mom's 55g) and the water from it (am getting a bigger filter for it today). the place i'm getting it from is actually a vendor on here (mike's killer fish), so there shouldnt be any prob's, but i would feel more safe in quar for a little while, esp when one of the fish in the 125 had ich about 4 days ago. i did take it out and none of the others have gotten any, so i'm probably in the clear.
i think a 10"er would be okay in a 20 for a couple weeks. maybe, ill just wait a few days to get the fish to make sure none of them get ich and then just put it in the 125. right now, i'm leaning toward the suggestion of powerfeeding a 5"er till its like 7 or 8". i'd rather spend less money and see one grow from a pretty small size then get one thats already half-grown. that way, i would be sure it eats well, anyway.