Not a terrible setup, though given what can be used in apocalypse the chaos side might be at a slight disadvantage since they will have considerably less power armoured bodies.
Well there is an update(?) coming out for chaos that includes a number of formations (under the black legion faction I believe, theres a thread about it you should look into for more info) so that might help.
Forging a narrative, not forcing
This makes sense, the predator assassin squadron datasheet came out before the implementation of factions, so theres nothing limiting it to loyalist marine armies. And there are a number of army lists out there for renegade guard, plus taking the current codex and using it as a renegade army.
Nothing wrong here.
Keep in mind this says datasheet, not formation or detachment. Thats because most/many datasheets were produced before factions (which formations and detachments use to limit who has access to what.)
Here is a list of containing chaos datasheets you could look into. They used to link to the respective sheets on the GW site, but have been taken down now, so you'll have to do the searching on your own.
http://www.heresy-online.net/forums/showthread.php?t=93560
Now from this would it be fair to say that this person could run chaos versions of current marine formations for eg a sky hammer using raptors and havocs , or a libby conclave with sorcs ( i know there is 1 in the crim book but its really crappy ) and run a a traitor legion with chapter tacs on stuff that is unmarked making it a chaptor on the turn is does this seem unfair/broken[/quote]
The formations, as long as you and your opponents agreed, I could see and doubt there would be much of a problem. But using a combination of chaos stuff and chapter tactics? That I'm less sure about.
If your going to use marine formations I would try to stick with ones that can make reasonable chaos swaps (like the librarius conclave or the demi company or the armoured task force, replacing the thunderfire cannon either with a second warpsmith or rapier battery.)