Hello.
Yes, you can use rims smaller than 18 inches (unless you have M Performance accessory brakes installed). Brakes are the limit: if you have basic brakes you can use 16 inch rims like the style you mentioned, but if you have M Sport brakes 17 inches is the smallest rim size (380 or 381 styles for example). BMW wheels and tyres configurators like
https://www.bmw.co.uk/en/topics/owne...and-tyres.html are currently down, but when/if they are up again you can use one as well.