I have a 500c and was wondering if anyone knew which pushchairs will fit into the boot?

A good retailer will let you try it first before you buy. All they want at the end of the day is your money. Really good ones will also be adaptable as child seats, again its all down to testing the various brands.

Because of how tiny the boot is there really is very little option, mother care should be able to help you by showing you the push chairs folded down but I think you're going to struggle to find something.

But I guess you already know this