I am a solid believer in 3 combat blocks which the crossbowmen could do in a pinch/late game. Just been drilled into me since forever. Plus it looks nicer.
I like a similiar big tercio of infantry, but prefer, where possible to have smaller parent units with detachments.
For this reason my 25 halberdiers are split into 17 and 7, with 7 crossbows added. My 24 man crossbow regt. Is split into 12 and 2 x6's . Drilled whereever possible. My regt of flagellents just fight as one unit of course.
But it makes for a very strong, very manouverable, very flexible battle line.
See my von Dufflecoats day out thread for more.