I understand your concerns. You want to preserve the integrity of the list.
Re: Limited spots on the list - Forbidding FCs who have multiple raid groups from posting more than one clear more or less defeats the purpose of putting this raid group based system in to place, as opposed to the previous FC/Linkshell based system. The whole point is to acknowledge the achievements of particular groups. This way if an FC has more than one group and a particular group achieves server first/second/third/etc for one fight but not another... it highlights that. I think it should also encourage people to take the competition seriously because they'll be able to really see the fruit of their efforts with this system.
Re: Integrity of kills and sharing strats/etc - As long as an additional kill for the same FC is 8 differing individuals from any/all previous kills from that FC, this is something unfortunately that I don't think we can really police because this issue can repeat itself in various forms ad infinitum. I'll demonstrate what I mean by this: "Did you get strats/info from your fc? Did you happen to be friends with someone on another server who gave you super awesome info or a private youtube video of a kill because you're not on their server and it didn't matter anyway? What if you and a friendly FC are working together to share info on the fight to progress faster? What if 2 raid groups that are friendly are doing the same thing and don't even have an FC?" As you can see in all of these examples, using your line of reasoning, we would need to invalidate every single one of these kills because they're not 100% truly organic in and of themselves. I think it's just too circular to ever really seriously police something like this. We'll need to monitor kills and use an honor system to maintain the list, but it's a pretty safe bet to assume that no FC group is going to seriously halt their progression on new turns just to go back and play someone else's characters just to claim spots on the list. I know my FC doesn't operate this way at least, and I assume that others would have the same integrity to respect what this list is and what purpose it serves.