Where are you getting one that small?
I had always thought they were larger than that at birth.
The smallest one I had ever seen for sale was about 12 inches. Most come in to the stores at around 18 to 20 inches in length.
Hell, if you can really get one that size, give me the name of the place to call. I would love to get one but like I said before I seriously doubt they can get one that size for you.
Anyway, as long as you leave ample swimming space in the 55 (not filled with LR like in a reef tank) you should be able to house it in there until the 250 has matured enough to move it.