I keep thinking about that rumor too. But honestly, I don't think Cassandra would let anyone be higher in military command than her... other than our inquistor.
I think it depends on how/why the group(s) were formed. It could be that they're sub-factions formed/assigned by the Divine, or maybe the leader of each created their groups as a means to deal with whatever their major issue is.
I don't think, however, that the red fists leader (be it Cullen or whoever) would be of a higher rank than Cassandra. The impression I got is that the leader is knowledgeable in that particular area and works with a military approach, for example: Cullen has been part of the Templars most of his life, so he's used to taking a militant approach, whereas someone like Leliana (rumoured to be the leader of the purple crows) is good with espionage being a bard 'n' everything, so her group deals with that side of things.
... If that makes sense?