In the past, I've been told building Fedora X would require the host doing the building to also run Fedora X. Not Fedora X+1 or Fedora X-1. Really Fedora X. It wouldn't be possible any other way. I assure you it wasn't my grandma or girlfriend telling me this.
Taking matters into my own hands, being the stubborn m-f that I am, I've been successfully building
on F7: FC6, F7 and F8t1 or rawhide
on F8t1 (rawhide): FC6, F7 and F8t1 or rawhide.
I've just not been able to build anything on X-1, but that's it. The rest, I did. And it's my very own application failing to do anything on FC6 because I've managed to create pretty hard requirements towards Fedora 7 or later.
Some of you think: "You didn't. It's impossible. I told you it was impossible. Period."
I'll add to that, that I've not only successfully built these spins -meaning that I didn't get any errors or warnings during the builds-, but they also successfully passed Q&A (although as we know FC6 had bugs in doing certain types of installations, so it didn't really /pass/ Q&A but it did anyway, and of course I'm not sure what Q&A requirements have been set for F8(t1)).