maintaining stability on a large stack is very difficult, real rockets get around the issue by gimballing the engines. but this game just kinda uses the imaginary rcs thrust from the sas modules and capsule. one of the mods contains an rcs module, that produces a lot more thrust for manuvering. but it doesnt prevent wobble very well. using one with an sas is a good way to stabilize first stage srbs, and if i use 3 meter tanks il stick one on the upper 1 meter to 3 meter adapter thingie below the decopler to the upper stages. on upper stages where you have say 4 tanks an lf engine, capsule, and not a whole lot else, i find you dont even need an sas or an rcs module, because you get almost no atmospheric interference to throw you off course. the capsule seems to come with an sas which is good enough to kill your rotation.
that said the modded parts arent very good. 3 meter tanks like to disconnect a little too easily. one launch left the 3 meter engine quad behind, i had to fix this by igniting the lf engine first, throttling down and then lighting the srbs. of course this can also cause an overheat which will destroy your entire stack. i presume the game developers did actual math to come up with values for tank size, fuel capacity, thrust, and structural integrity, etc, where as modders just fudged the values.
another game where you can assemble a space ship (thought he focus more on combat maneuvers) would be space combat, which is a lot more intuitive. lets you configure each part's size, and dimensions, but then goes and correctly computes its mass for you. of course space combat is also a lot less fun, despite the fact that you get to shoot things.