Mixed Crew

A "Mixed" crew classification on an Alien Sub means that there will be a mixture of multiple alien races on missions associated with that Alien Sub. There are two subtypes of Mixed Crew designations (race types 4 and 5 in the game files), though you cannot tell which a given Sub is on the Geoscape:

Type I: Primarily Tasoths with Bio-Drones/Triscenes (on land) or Tentaculats (underwater) for terror units, with some Aquatoids mixed in and a tiny number of Xarquids (on land) or Hallucinoids (underwater).

Type II: Primarily Lobster Men with Bio-Drones/Calcinites (on land) or Tentaculats/Hallucinoids (underwater) for terror units, with some Aquatoids mixed in and a tiny number of Deep Ones (on land) or Calcinites (underwater). On a Dreadnought mission underwater there will also be a singular Deep One, the only time these units are found underwater before T'leth.

Mixed Crew missions tend to show up late into the game, so they can be a backpath to obtaining a Deep One or Calcinite if you missed one during the early months when they're abundant.

Mix of races in Alien Colonies

The way alien bases are crewed in TFTD differs slightly from UFO. Where UFO bases will be manned and supplied by the same species as the construction crew, TFTD colonies are operated with a mixture of alien types.

Alien colonies are not manned by a mix of crews in the same sense that the mixed crew USOs are. Instead, they will have a unique selection of crews for the supply runs, who defends the topside section of the colony, and who will man the interior of the colony.

The crew of the supply ships will be the same as the crew of the aliens that established the colony. So if Gillmen do the scouting and construction of the colony, they will also be responsible for the supply runs.

The topside of the colony is always manned by Tasoths and Aquatoids, accompanied by Tentaculats and Hallucinoids.

The interior section of the colony is always manned by Lobster Men and Tentaculats.