And if so, is it possible to scale them in relation to each other?

Let'ss say I place 4 barrels next to each other, each double the size of the other. If I double the size of each one, they'll all be clipping together. Is it possible to change the size of things in relation to each other so that they'd stay "next to each other" instead of going through each other?

Of course this all depends on whether or not I can scale multiple things at once

Not that I know of, but you can copy and paste the number representing the reference's size easily enough.

Nope. Scaling ignores any distance between specific objects, and just scales relative to the centre of the reference.

You could select all 4 barrels, then press and hold the "S" key and scale them together. You would have to manually reposition them.

Boooooooo.