This power doesn’t trigger off an ally entering play; it triggers off an ally being played. You choose whether or not to put 1 damage as that ally enters play. If you do, you draw a card before any player gets priority. If such an ally is interrupted, you can’t draw a card because the "if you do" fails. If you control two Dimzers and play an ally, you can put 2 damage on it as it enters play and draw two cards. However, if that ally has 1 health, you can't draw a second card because you can’t put more than fatal damage on it. Similarly, you can’t put damage on an ally with printed health 0.