I too will be joining this, I have a CS in the works. 

Just to avoid overlap I'm currently designing the Xth legion, focused on lightning war and fast attack style tactics. 

As far as overlap, I don't really see too much a problem with it. Even in canon there were often parallels between Legions (usually between Traitor and Loyalists) in both style and tactics, so having overlap may provide some interesting interactions between Legions.