the amount of chaos bitz you add will depend on just how far-gone your fallen warband is!
some of the fallen have truely gone over to the greater gods, so in that case you could include daemonic minions like the possessed, lesser daemons & defilers.
for these warbands, you can also go to town on adding in chaotic bitz and include various marks and cult units as well!
however, many more of the fallen realise that they were wrong, so if that's the case with your warband, then i'd limit your force to just chaos marines and no marks or daemonic/cult units of any kind.
i'd also keep the chaotic bitz to a select few, such as the older style armour and re-inforced shoulder pads. avoid any overtly chaotic iconography as these guys don't really worship chaos at all - they're more like outcasts and/or mercenaries who are on the run from their former bethren!
cheers!
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"Who are you to question my methods?! A 'Puritan' is simply a coward who hides behind a veil of pretended faith and denounces his loyal brother as a traitor out of simple jealousy!"
- Inquisitor Odion |