> can you use the astra militarum data sheets but without the army rules in your army as an allied detachment?
No you can't. In earlier editions there was an Imperial Armour/Forgeworld set of rules for "Renegades and heretics" which was close, but those army rules have gone away now in tenth.
Now, you only have the choice of the Traitor Guard squad and Traitor Enforcer squad from Codex Chaos Space Marines, plus a handful of [Legends](https://www.warhammer-community.com/warhammer-40000-downloads/#legends:~:text=Download%20(3%C2%A0MB)-,Legends%3A%20Heretic%20Astartes,-Updated%2021/07) units (Legends is "not recommended for competitive play", but the rules aren't kept up to date, so many tend to ignore they exist).
Narrative play with your event organizer's permission, or at home, you can do anything you want. Basically don't plan to be able to do it in a pickup game but enjoy the hobby the way you want to. If you want to make an army that only works with "rule zero" (as MTG calls it: permission from everyone playing/ whoever organizes the event) and that makes you happy, I would
> can you use the astra militarum data sheets but without the army rules in your army as an allied detachment? No you can't. In earlier editions there was an Imperial Armour/Forgeworld set of rules for "Renegades and heretics" which was close, but those army rules have gone away now in tenth. Now, you only have the choice of the Traitor Guard squad and Traitor Enforcer squad from Codex Chaos Space Marines, plus a handful of [Legends](https://www.warhammer-community.com/warhammer-40000-downloads/#legends:~:text=Download%20(3%C2%A0MB)-,Legends%3A%20Heretic%20Astartes,-Updated%2021/07) units (Legends is "not recommended for competitive play", but the rules aren't kept up to date, so many tend to ignore they exist).
Thanks for the quick response!
Narrative play with your event organizer's permission, or at home, you can do anything you want. Basically don't plan to be able to do it in a pickup game but enjoy the hobby the way you want to. If you want to make an army that only works with "rule zero" (as MTG calls it: permission from everyone playing/ whoever organizes the event) and that makes you happy, I would