It's not a good thing. I don't think it's dangerous.
Yeah, I was thinking interference with the brake calipers from the smaller wheel.
But if it fits without any interference when moving, then it would be OK.
Carrying plugs and a compressor is always a good idea, in my opinion. I bought a plug kit some years ago, and used it twice on my old Outback when I picked up nails in my community (reshingling houses, I think), and once when Castle Key picked up a screw in his one tire.
Which reminds me, I need to go by the auto parts store and pick up more plugs.