I don't think the FFV forum gets enough traffic to stand alone, but other than that, I don't really have any strong disagreements with the current setup. I understand why FFVI would be a standalone, but really, if IV isn't good enough to be a standalone, then neither is V. Put V in with 1-4 if that's a better idea, or put it with 6. Any forum it's merged with isn't going to be overflowing with FF5 topics, because the FF5 forum itself isn't overflowing with such topics.
