This is an off the wall question I know
But.
The harmonium conducts a lot of its training in Arcadia, and yet Arcadias infamous for a Kill on sight attitude towards those of Evil alignment.
Ideas on how a Baatazu who joins the faction is handled, do they ommit the take them there part, and expect him to allready know how to make war? (which he doubtlessly does, just a reminder to "Capture them alive!"
The Harmonium's baatezu recruits might well be part of the reason one of Arcadia's layers slipped into Mechanus.
They probably just take their baatezu recruits directly to their secret training camps (through portals) and avoid the Arcadian petitioners and natives. If Arcadians knew the Harmonium was letting evil creatures into their plane... there'd be Arcadia to pay. They wouldn't be happy, is what I mean.