Even though Metallic Mimic is a Shapeshifter, other Shapeshifter creatures you control won't get a +1/+1 counter unless you chose Shapeshifter as Metallic Mimic entered the battlefield. If you do, that creature must be entering the battlefield as a Shapeshifter to get a +1/+1 counter. It won't get a +1/+1 counter if it enters the battlefield as a copy of a non-Shapeshifter creature.