But they'd eventually run out of pila, then things would get fun... A maniple had a frontage of 20 men, which at about 1.2m per man makes 24m per maniple. The gap would also be about 24m. That's quite a large gap for people to be pouring through, I'll give you that...

Really good ideas here, I'm glad my question has gotten people thinking. The sidestep seems to me a reasonable assumption with regards to the fighting... If only we could get a bunch of reenactors to wargame this.