Note: I focused on getting the screens functioning, I will make them look pretty and more informative later.
I've finished the screen that will pop up when you click on "Transport People" to put people into your fleet. This is what it looks like:
If you select too many people to put on board, it will gray out the transport button on right. You keep adding people, but you can't put them back (to avoid exploiting by moving people in the same system). You make adjustments to how much of each race you want to take on board, then click on the transport button to confirm.
When you order some or all of your ships to a destination, and the fleet have some people on board, it will process whether or not to pop up the split population window. The following cases are when it won't pop up:
If all of the transport ships are selected for the new destination
If all of the transport ships are left behind
If the total capacity equals the total population on board, and there's only one race.
Otherwise it will pop up and let you choose how much of each race to go on each fleet. This is what it looks like:
Note that I don't have a transport ship selected in top, and I didn't fill it to the maximum capacity, so the prompt popped up when I right clicked on a destination. On the left column are the amount of each race to leave behind in the unselected ships. On the right are the people to put in the ships moving to the new destination. The confirm button will be greyed out if one side have too many people.
Next up is landing of those people, and ground combat. Is the population transport system clear, or are there some aspects that I need to clarify?