What other fish are in the display? At little over a month, any hermits, crabs, and some corals could make an easy meal of them. How large are they, an image would help.
It would be best to wait until they are 3/4 of an inch or bigger but it all depends on what you currently have in the tank as well. You do not want them to get picked on.
I would put them in and monitor closely for several hours to make sure they don't become prey!