Generally you'd want the plaid and kilt to match as much as possible. That's why whenever I order a fly plaid for someone who bought their kilt years earlier I try and get as much information as possible about the kilt (what mill wove the tartan, what weight range) so I can order from the same source. Even that is no guarantee, though.

It has certainly been done in the past to wear a plaid in a different sett size, or even a different color scheme, than the kilt, however.

The classic example for me is the Atholl Highlanders, who wear kilts in the modern Murray tartan, and plaids in rather faded, nearly 200 year old Murray tartan. The effect is striking!