Can you use 15 inch wheels on a Grand Caravan that has 16" on it already

Sure- if the bolt pattern is the same- but if you want the speedo and odo to stay accurate, get a higher profile tire than what the 16s have- for example use a 75 series instead of a 65 series- this will also have the benefit of a smoother ride-

you can go up or down 1 size or inch....

Yes , except for throwing off the speedo, clearance and trip, like the other guys said, but my question is why? Your are going to have a much better with 16''.

No. There is not enough clearance for your brake rotors.

Will it not fit for some 15 inch rims or all 15 inch rims ?