The answer to this question was provided as an answer for the question linked in my original post. I have copied Hubba88's answer and Zommuter's comment here for consistency.

March: Sectoids, Thin Men, Outsiders

April: Floaters, Chrysalids

May: Mutons

June: Cyberdiscs, Heavy Floaters

July: Muton Berserkers

Alien Base: Drones, Sectoid Commanders

Overseer: Muton Elite, Sectopods, Ethereals

Slingshot DLC Mission 1: Thin Men, Chrysalids

Slingshot DLC Mission 2: Mutons

Slingshot DLC Mission 3: Cyberdiscs