Previous codexes (haven't read the current one!) have justified Rubrics being in non-Thousand Sons armies by saying that their leading Aspiring Sorcerers are very mercenary. They'll join up with other forces and warbands to fulfill their own purposes, whether it be for wealth, power, influence, some kind of legitimate alliance or just good old fashioned spite. Since the Rubric marines themselves are essentially automatons they'll just go wherever their master goes.
Having full-on proper Rubrics that have nothing to do with their parent legion is a bit iffy. I think a snippet of background somewhere says the Black Legion have managed to replicate the effects of Ahriman's spell, so it's not TOTALLY unreasonable but it seems like the kind of thing only someone with incredible resources and cream-of-the-crop psykers could pull off. You could go for a "not-Rubric" angle, where a Tzeentch following squad of marines is infused with the sorcerous power of their god but need a psyker to hang around with them to control the magic and make sure they don't explode into Chaos Spawn or spontaneously combust or suffer some other terrible magic-related fate.
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