1) The extra fee is for an extra IP address. Since you are using a router, you only need one IP address, and AT&T has no way of telling how many PC's you have hiding behind that router.
2) Nope - as long as the router is set up as a DHCP server (which they usually are), both the Mac and the PC should pick up IP's and get out on the internet just fine. Now if you want to start sharing files between them, well, that may be a whole other issue.