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If you weren't playing a tournament game and you had a group of friends who didn't mind any of the Horus Heresy additions (Praetors and the like)

Could you essentially ally your Post-Heresy army with a Pre-Heresy one?

Sorry for all these question threads too. I'm sure they're getting annoying by now, but I genuinely don't mean to be a question spammer. I just have a lot I'd prefer some information on before I go out buying models to represent this and that.

Pre-heresy marines have their own allies table don't they?
I don't think so

If you had a group of friends who didn't mind, you could be playing 2nd edition with completely made up units.
If you had a group of friends who didn't mind, you could be making up entire new codices and units and allying with them.

On the other hand, why are you trying to build a "This army would only be valid in the event of time travel" army?

I'm saying more along the lines of Pre-Heresy Emperors Children with Post-Heresy Salamanders for example.

But of course, it's all just fantasy anyway.

EDIT: Or even creating a new Chapter with those rules/equipment/fluff and then allying them with a Post-Heresy chapter treating the former as Legiones Astartes (which of course they would be).

New Chapter X (Led by Praetor A) and New Chapter Y (Led by Chapter Master B)

Something like that.

If your friends are cool with it, do whatever you want. If you as a group decide to change the rules when you play each other, that's fine.

Short answer no, long answer is no because the allies matrix for legions does not include any codex other than the legions list.

No, no, no.

The question does not really fit here, as the YMDC forum is to decide upon how rules work in the BRB and making calls upon confusion.

Are you trying to clear up a rule?
Do you not understand a rule?

Or,

Are you asking if you can play WH40K with a friend of yours at his place with your own made up rules?

Because if its the first 2, we can continue. If it's along the lines of the third option, i'd say, why of course you can do whatever you please with whoever you want, but it does not belong in this Forum...