Basically:
Marines of any armour type = Marines
Terminators of any armour type = Terminators
But I don't think there are different rules for different Power Amour or Terminator armour types.
I'd use the Chaos Space Marine rules for Pre Heresy marines. They get access to all the right stuff and you can just use the Marks to differentiate Legions (I was going to use Tzeentch for Iron Hands Bionics, etc). They're the Kill Team List that has primary access to Power Armour, Jump Troops, Terminators. The Cultists can work as Legion serfs but I'm not sure what you'd count the spawn as.