But by this point in history has become a derogatory term also in Greek, it can be found both in demotic poetry and in texts being used as an insult. It's hardly a good idea to name what amounts to an army sized sized formation, "army of the apostates ", worse when most of the soldiers involved do know the term as an insult. So would suggest something else. Although given their size at over 20,000 a regimental name will be hardly practical, this is an army sized formation here.
Actually I think that would be a division.