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I have quite a small space marine army, and the kill team cassius, I was wondering, can my duel powerfist captain join them, as I don't have a squad that he will be good in

Counts as watch captain?

I think they are still battle brother level allies. So yes, with a few caveats.

Unless you go unbound, you captain will need 2 troops, or some other formation/detachment on his own.

IIRC the new WIP FAQs prohibit allies from starting in transports. Don’t think that’s an issue here.

But when deploying on the table, you should be able to add him to the squad.

(Full disclosure, I don’t have a copy of the DW codex, but of all the commentary I’ve read about it, this issue was never brought up. Or at least none I can remember before my fist cup of coffee.)

They're Battle Brothers, so yes the Captain can join the Kill Team (and the Deathwatch characters can join the other Marine units).