Rival Domain, will it be 4v4/muitiple maps or 5v5 or higher/one map

Simple answer, we don't know. All we can do is speculate until the update.

I think in the notice they said you could bring up to 20 units(either yours or your friends), the only restrictions being only one dancer/sing per team and no duplicates

Thats a really nice Arden, I added you, I am the one with the +7 Tharja.

(EDIT: Removed, I thought Follow-Up Ring only worked when initiating)

Also, replacing Aether with Noontime means that your Arden will never die by melee attacks. He heals with every counter attack. This is all up to you though.