No, it's possible in some areas to have your rims and tires be bought separately, but that all depends on where you go, and the availability of that service. It's easier to just replace them both at the same time.
If the seller has more than one of a particular item for sale, that information will be shown in the listing.
When you use the "Buy It Now" button, you will (on the next page), have the option to...
Yes, rims can generally fit a tire that is +/- ~1 inch (25.4mm) the width of the rim safely, some people stretch this to ~1.5-2 inches but ill stick with the "safe" tires.
So ill break it...