Devil's advocate argument: there's nothing to say that the Faction symbol indicates that the Formation can only contain models from that Faction. There is RAI to back this up, as well: the Admech War Convocation Formation has the Cult Mechanicus symbol, but contains units that are not found with the Cult Mechanicus Faction. Similarly, the Blood Oath supplement lists Formations that have the KDK symbol, but contain units only found with the CSM Faction (i.e. Brass Scorpion, Blood Slaughterer, Decimator). If you want to argue that this symbol is binding, then it makes it impossible to take any of these mixed-Faction Formations. Now, I don't agree with this interpretation, however there it is for the sake of a balanced debate.
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