Good idea, don't gripe or it will show the dealer your ignorance. It states clearly in the printed and online versions of the Expedition and EL brochures that with the 18" tire ordered, you will still get a 17", full size, spare.

Because I do tow 5400 lbs, and do so in way out of the way places, I ordered 17" wheels and tires all round because of their availability and lower price. The Expy came with all season Contitrac TRs and the spare was a Contritrac SUV. Same size and specs, still an all season, but slightly different tread pattern.
One inch difference will make a difference when on one tire only, for the functioning of the ABS system/antiskid/antirollover system, and in rear diff lubrication but really shouldn't harm the vehicle over a few hundred miles until you can find the right replacement or repair. Not a good idea for a permanent installation however for the aforementioned reasons of system calibration/performance problems.