Strangely I find vanilla Yari Ashigaru quite historically acurate, although you're right and it looks ackward when they place those pikes on their backs (maybe they have extra slot for them, just like that for sashimono?
).
As none of those things affect actual gameplay, but only changes weapon models and animations, it's a matter of personal preferences (and what bothers you more when watching battle animations). I would go with vanilla setting, or ashigaru having short yari only (no sword), as it doesn't look that weird in close combat.