I was answering the first half of your question that there is no reason to transfer MilTeeth to any of the Mil3 figures because all those figures already have the teeth.

For any other figures, your best bet is resizing and parenting the teeth figure to the character's head. That allows retaning the figure as a character that can be posed separately from the figure's mouth. Using an Obj prop will leave the teeth stuck in whatever posed you saved it as. If the character, you're putting the teeth into, has a separate jointed jaw, versus mouth morphs to open and close the mouth, saving the top half of the teeth, and bottom half, separately, allows parenting the upper to the head and the lower to the jaw.