For a while I have thought about remaking my chaos renegade warband into a 1st founding legion. The CSM book is a mess, trying to represent too many different factions with the same units with no clear structure for any one of those factions.
I decided to go with the Alpha Legion for a variety of reasons, and from what I have read, they use stealth tactics, cultists and are all proficient in any role (assault, heavy weapons, terminator armour use etc), but I am not at all an expert on them and I have no clear idea on what their actual unit focus would be.

Which units would they use and how would a fluffy army be organised? And which units would be a no-go for them?
I dont have any problems with the army not being super competetive, I would much rather have a fluffy list and then trying to win with that, than just using "good units" to win.