In keeping with my longstanding (though not often put forward) regimental fluff of being Chapter serfs to a successor chapter of the Reasonable Marines, I'm theorycrafting an allied detachment along the theme of operator Marines to slot in. Damn, a Sternguard bomb is expensive, though. Might have to just make do with a five man unit instead of combat squads.

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Allied Detachment

Librarian - 65

Scout Squad - 84
-Camo cloaks
-Sniper rifles, missile launcher

Sternguard - 355
-Five additional Veterans
-Four combi-meltas, four combi-grav, two heavy flamers
-Drop pod

Then again, the reason I'm thinking of doing this now is ability to eliminate things like that damned Supremacy armor before it wrecks my entire army. If I'm going to be seeing that regularly now, it may well be worth it.