1. You can't make it so you HAVE to, but you can make it EXTREMELY inconvenient not to - say you want highliners to be buildable only in space - then give planetary space yards 1 build rate in rads and make only highliners use rads to build! Or just have "gradations" of spaceyards which have vastly different build rates and costs, so it's impractical to use a micro yard to build a highliner but wasteful to use a mega yard to build a scout!
2. AI tags perhaps?
3. I don't think you'll reach this point - SE4 is quite moddable and the community is quite innovative
Re: A mod idea :)
Threaded Mode