Royal Stewart tartan is kinda the property of HRH Elizabeth II by default, she says that this is a universal tartan so anyone can wear it. There is a IIRC Balmoral tartan that you can't buy that is effectively restricted to the Royals. Any tartan that you can buy is fair game but some clans do act like you are the worst sort of sinner to wear "their" tartan.

There are dozens and dozens of tartans that are "Universal" and only drunk guys at Highland games will give you greif for wearing them because they will mistake them for their own tartan. But this happens no mater what so be happy wear a kilt in the tartan of YOUR choice.