roles change which other roles they target, and should also change where they spawn in combat. Ships will still shoot whatever comes into range first or at least they are supposed to. I normally quick battle all combat, but have heard stories of battle viewer achieving different outcomes.
there is no global tax rate, so it normally is a good idea to build approval on a planet before building cities. also a farm only makes 3 food, cities take 4, so you need 3 farms for every two cities. silicon based i don't know costs.
any constructor building a shipyard uses all remaining points and consumes the constructor. in theory building a new starbase should only take 1 construction point, so constructors could still build multiple. however with the modules taking administration points at least in crusade, i usually just build single constructor w/ lots of engines and range.
as for the free building, it basically only works with regular buildings, not wonders. I am unsure if it still works with shipyards or not, hub buildings like the space elevator might be a good use, tho they are low cost to produce anyways.