You can mix. Think of the Angel of Mons. Alongside the early war British forces you had Longbowmen led by St George. You could even go even a little bit madder and add in a few Martian invaders-.
However, this is the Holly King’s day - the Dark (winter) Lord reigns. He is the god of transformation and one who brings us the birth new ways. The bad god.
Do not get mixed up with Satan or Lucifer, god's civil servant who sits on his left hand.
The representation you might know best is the Green Knight in the Arthurian myths. Set fits here. Use your hairy arsed Celts, nasty Vikings, Panym Sarecens who worship Jove, Judean National Front (as the Romans are on the other side, don't read anything into this, you can always swap them!) and any others you can think of.
Well that's it, short and sweet and a sort of challenge for your clubs seasonal bash.