There are lots of threads about this. It seems to be on a store by store basis. My store accepts KC even it it's very old. But some stores accept if it's been a short time and other stores don't accept it at all once it's expired. Call the store that you shop at and ask. If they say no, call around. Maybe you'll find one that does.
There is no minimum purchase but if you're using a coupon, they take the KC off first and then give you the discount on the remaining difference.