Say I summoned a pack of Seekers of Slaanesh 12" in front of me, after moving 6" on from my 12" deployment, could that unit then charge, if I went second?
I would say when you summon it that as it is bursts out and possesses the flesh of the chosen victim that is isn't flying.As another side note: Flying Monstrous Creatures must come in from reserve Swooping, so if you summon a Bloodthirster or Lord of Change, it can't assault the turn it comes in or your turn after it comes in, as it has to change flight modes that next turn so still can't assault.
Oh how I wish. The rulebook is pretty ironclad about DSing FMCs, though. So, RAW...Equally that is where the throat punching rule comes in with the mighty words of "Stop being stupid".