The benefit of adding more adult bluegill over the winter may be tilted towards relieving the source pond of 'overcrowded' fish which may help general over-wintering survival in the source pond.

The advantage one mostly tries to gain in the new pond is having bluegill in there before their spawning season. They'll pretty much be close to dormant over the winter, before the spring of next year.

Also, you have to be sure that what you are stocking are the actual bluegill that you want, and not some variant or cross.

Those bluegill from the source pond may be stunted, but I don't think that's really too much of a big deal.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."