The only concern is the width of the spare rim as compared to the width of the regular wheels. All tires are designed to be mounted on a limited range of rim widths depending on the width of the tire. If you mounted a wide tire on a narrow width rim, you could be creating a safety problem in that the tire bead may not be sitting properly on the rim flange. You may also create a handling problem with this conbination.
If this is the case, your donut spare tire on the spare rim would actually be a safer alternative.
For my 740, I found a "narrow" tire that had the same diameter as the regular tires and mounted it on the narrow spare wheel. This way the tire fits the rim properly and it will still carry the load.