User blog comment:Trulyrandom/Are Fanon Septs NCF?/@comment-3258762-20130813135127

The major reason why 2nd founders are not allowed is because, if they were, we would have hundreds of 2nd founders. Due to the 2nd founders being active around the time of the Great Scouring, it would mean the Legions would have had enough troops to form so many 2nd founders after the HH, meaning it would not fit that they actually had so much problems with the Traitor Legions.

I believe the "All 2nd Foundings are known" was added to ensure we had even more backup to enforce the rule.

With Tau Septs, there isn't really the same dilemma. No one is going to complain if we get another 1000 Tau or the like in the galaxy.