Can you use an 18in spare tire with 20 inch rims on the other tires?

If the 20 inch are the original factory size, then yes, as they have the same rolling circumference.

That said, the narrower tire won't handle the same, so watch your speed and don't push it anywhere near a cornering limit.

To the owner that posted this photo: Can you PLEASE measure the top of the scissor jack. I meaning the rectangular top that engages the jacking points along the sides of the vehicle. Here is why. I want to see if an X5 jack will work on my E90 335 sedan. They sell this querky (in my opinion) jobs that work on the sedans that are known to fall and are unstable. I have been using a scissor style from my Honda to lift up my 335D. I want to have a long term solution while on the road and think the X5 scissor will work. What i don't know is whether the X5 E53 or E70 jack point (the little plastic rectangle thing) is bigger than my E90 one is.
thanks