It would be great if the neutrals obeyed the political and cultural settings, higher xenophobia reducing their desire to be transported closer to alien home systems, and reducing the amount who would want to be transported to any planet with populations of a different species, determinism effecting their spread to places that are troubled, perhaps anywhere that's had battles, or dust/ radiation, or near enemy systems, trading effecting their spread based on colony wealth or raising the spread towards alien worlds, diplomacy effecting spread towards alien systems, or more likely towards multi cultural or multi species worlds, also towards areas that have recently seen war, expansionism causing expansion towards the furthest reaches of space or anywhere with unlimited population aka cost 1 worlds, and finally militancy causing spread towards areas of unrest or warfare, or perhaps otherwise reducing their desire to leave, instead wanting to stay and fight.
I can see a system where when a ship arrives to pick up colonists the final destination is checked against all populations, they get a bonus or reduction calculated which then affects the ratio of colonists drawn from each population the ship then picks up a bunch of all populations and dumps them on the selected planet. The important part is that when they arrive each group is added to that planets population, all acting as one colony, but the racial make up of the colony is still kept track of, in the civilian tab underneath the colonisation status selector it could list the different populations that make up the colony.
