I hate to be that guy, but this is wrong.
The solar system is mostly in one plane because it formed from a cloud of gas. The cloud of a gas has some total non zero rotation and as the cloud collapses interactions flatten the cloud into a disk, where all of the planets formed.
This same principle applies to galaxies.
The original question was why solar systems and galaxies are in planes, and your explanation is wrong.
What do you even mean by similar orbits? Most orbits are circular for a totally different reason, and that is tidal interactions.