I haven't worked out the rules yet. CP is on the list of things to consider. One thing for sure: we will be using the narrative method for building armies. So you can use detachments or not as you see fit.
The last I heard is that Chapter Approved is scheduled to drop in December. Whether or not it'll have Apoc rules is something I don't know. If there's a resource available before the event we will use that. Otherwise I'll cobble some stuff from the old Apoc into an 8th equivalent.
The Apoc rules are apparently going to be in the Chapter Approved book. Go nuts making whatever rules for this one rob. I think the 2 big things is CP limit (if any) and also psychic powers and if we are gunna use matched play restrictions or narrative rules which allow any power to be cast however many times. Personally curious for the ruling of the latter.
"How ironic it is that, as fast as we spread progress and hope throughout the galaxy, the Tyranids spread death and despair." -AUN'SHI OF VIOR'LA