Rulings

• The choice of creature type is made as Metallic Mimic enters the battlefield. Players can't respond to this choice. Metallic Mimic's second ability starts applying immediately.2017-02-09
• 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.2017-02-09
• You must choose an existing creature type. "Artifact" and "Vehicle" aren't creature types.2017-02-09
• Creatures of the chosen type that enter the battlefield at the same time as Metallic Mimic won't enter with an additional +1/+1 counter.2017-02-09