[Randallw]

I find it rather complicated. As you know you can get other races to migrate via treaties and they will head for your planets with their atmosphere, however your people who don't breathe that atmosphere (likely because they colonised it beforehand) won't leave. In one game I was quite prepared to let them be removed, as I RP. If they were robots or a hive mind I just space the excess, however with emotional races I try to plan their removal. It's damn hard. You need a ship big enough to take all of them, and it tends to remove pop and fill the ship while leaving their children still doming the planet. In frustration I decided RP can go far enough and I started spacing the excess.

When you have a treaty of migration it does not just cover the ally race but all the pop they have. If they are also accepting migrants from your enemy those pop can be diverted to you.