With Crusade, I always just have a dedicated farm world to feed all my other planets. Specialization is the way to go, and focusing your efforts on one agricultural world allows you to pool Farmer citizens, if needed.
I don't know about a specific build order early on, but I typically like to have my main planet a production world so that I can get some constructors/colony ships out there. I adjust this at times if my homeworld just ends up being terrific for research/credits due to tile bonuses.
I did have a problem with "overspecialization" in a somewhat OCD way at first. My first games I'd forego building the computer core or central bank because I should "save" those for my research/economy planets. They're so cheap and easy to build, though, it's better just to get them down ASAP. You can always destroy the building and rebuild them on a new world.