They will definitely grow and grow fast as long as they have plenty to eat. They will feed all winter, just not as fast as in warm water.
As far as a fence, if TPWD didn't ask you to build one, I don't know that I would. If your pond overflows more than six inches deep over the emergency spillway, grass carp will escape. But, if you choose to build a fence, put it somewhere downstream a little bit so as not to compromise your dam. I have seen several lakes blow out because of a fence across the spillway. Make sure the top of the fence is at least a foot below the lowest point of your dam.
