I feel for you.  This has been one of my major Carrara peeves since it first came out.  Scales in all of the various modeling rooms and the assembly room SHOULD by default be matched, but for some reason known only to themselves, Daz and the previous developers have left it this way which is annoying as heck.  It should be one scale.  If you change it in one room, then you change it in all of them automatically.  I can not think of one single reason why someone would want to have different scales in each room...  It makes it very difficult to get the relative size and scale the same between different objects without having to resize them.