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If I were to make a TS army, would it have to be composed of just Thousand Sons models, such as Rubric Marines, or could I also field regular CSMs?

Also, can the Thousand Sons hold their own without Magnus?

As long as your detachments are thousand sons models, you can ally with CSM.

Can they hold their own without Magnus?

I have had a bear of a time against horde type armies. I simply don't have the model count.

Especially against GSC that recycle their units.

You Can use any CSM models as long as they take the mark of tzeentch. The mark is probably the weakest one tho for units without an invulnerable save already.

I think there are plenty of competative builds without Magnus but you'll have to work a lot harder in list building that other legions, and make good use of allies.

You can run a TS detachment and use any models that don't have the mark of Another god. Every unit that can, will have to take the Mark of Tzeentch.

As others said, the rule simply states that if a unit can take the Mark of Tzeentch, it must. And it can't take a unit that can't be marked.

As the above poster said though, the MoT is easily the worst of the marks for pretty much anything that isn't an HQ. The exception to this being Terminators. A large unit of terminators built for assault with Mark of Tzeentch along with a terminator lord with the AG would be very powerful.

People seem to do well without Magnus. I haven't won a game yet but I'm playing only 1000pt games which is not ideal for Thousand Sons.

A normal Thousand Sons CAD using the Legion book can take whatever you want, it's your usual CSM list with extra detachments and a few extra stipulations (all tzeentch no special characters other than Ahriman).

Now if you want to take the Thousand Sons Decurion formation, then you're stuck using only rubric and scarab stuff, since all the Thousand Sons formations use those exclusively.