Will all depend on the offset I think. I've got 10" rears ET40, but on 19s, but with a 40 profile tyre, 275 width. According to willtheyfit.com that's actually 2mm larger diameter than a 20" with a 275 but 35 profile tyre.
I reckon therefore that you could go to 10" width and still be ok as long as your ET aid ok on the wheels.
Willtheyfit.com is great for trying different combos