It isn't completely ahistorical for the janissaries to wield polearms actually, members of special assault squads (called 'head riskers' and made up of volunteers from the Azaps and Janissaries) may have used polearms, a sort of staff weapon called a Tirpan(I think). Though janissaries actually used what weapons they preferred, which was usually a composite bow and a small close combat weapon, though these assault squads may have used polearms since they were usually used in siege assaults or for charging enemy formations and then you might need something bigger than a short sword or hand axe.