The whole idea already takes extreme artistic liberties, loosely combining Arthurian myth with dark ages history with the seven samurai idea, so what's a little push further? Once the idea formed...well, you know what they say. Spreads like a disease, hard to kill, and all that. So, now, Roman/Celtic/Angle/Saxon warlords are still present, just...singular. I thought I might as well add a legion from Africa, and may as well draw some inspiration to have Persian mercenaries as a sixth. Not sure on who the seventh would be, would have to check my blog posts prior on the idea, not to mention, start actually doing research.
For an idea like this to be viable, I'd have to have a better understanding of what tribes existed where at the twilight of the western Roman empire. But I do think the idea would be interesting to pull off. (Fun fact: the game that I made a few weeks ago? Which I blogged about at some point? I actually invented it for this story idea, so I have been thinking about it some.)