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Yes. Then watch them when you put them together, to make sure the big ones don't trample the little ones, or pick on them too much. But at those ages, with 3x as many little ones, they will probably be fine.

^ Exactly what I was going to suggest :) Wait until the younger ones are steady on their feet and running around well (a few days), then put them together. You shouldn't need to do any complicated introduction, just plop one group in the pen with the others. They can be in two separate pens...