Not for each soldier type, but for each soldier type group. The crafts would still be restricted to their respective races.
If you'd construct a terror_ship_for_ethereals (this ship would have allowedSoldierTypes of ethereal soldier, ethereal leader, sectopod, etc..), then sectoids wouldn't be allowed on this craft. But if you added a manufacture design that takes terror_ship_for_ethereals as material and produces terror_ship_for_sectoids, then it wouldn't be a problem. Well, besides needing a lot of manufacturing designs for all combinations heh.
So restricting a group of soldiers for specific crafts would help more modders than having tags directly placed on soldiers themselves then? Fair enough, I could work around this restriction, I aim to make races a meaningful strategic choice anyways, the only reason I wanted this feature to allow for any race on any ship was mainly to be less restrictive at this level of play, but it doesn't hurt going deeper.
If more people prefer craft restriction (all the way as you propose here), I'm perfectly fine transforming that request to fit that criteria for sure!
It's a good call in the end anyways since I have alien ranks (which aren't "ranks" per say but different soldiers [Transformed, just have the rank in their name+stats really]). I could prevent certain soldiers (like terror units) from crewing a small scout ship for instance with your idea, since it's basically a list of allowed soldiers. Nice catch!