I am not following:
Can't SU register if one is stretching from the left or right end of a line?
If the line is connected, can you lengthen or specify a length and answer "No"; (Do not scale the whole model)?
If objects are connected, grouped, or components, can they be "separated" temporarily to work on specific dimensions? ("Scale" is great, unless that is not what you want)

I am a newbie practicing on a shared model with a barbell weight set:
The weight plates don't seem to have inference guides (center, diagonal, radius) to help measure. ?
Objects are already grouped (components? The language is not parallel and doesn't make sense). I copied one plate off the bar and can grab and move it independently; though, it remains a component, and I can't scale it to my specified diameter.