I am also concerned that you can't fire the BC and PC's in the same phase. My reasoning is that Lumbering Behemoth allows you to fire the Turret in addition to other weapon/s. The current rules allow you to fire an Ordnance weapon whenever you like, but causes all others to be fired as snaps. You can't fire a blast as a snap shot.
The wording in the Rulebook is such that firing ordnance causes other weapons to be fired as snaps, not that firing blasts as "normal" prevents firing Ordnance. Splitting hairs, but a distinction is there. You could say that LB allows you to fire the turret, even though firing a PC would normally prevent it... so LB works... except LB doesn't change the core rules surrounding Ordnance causing all other shots to be snaps, which is what prevents the PC's from firing.
In essence, Lumbering Behemoth allows you to fire an Ordnance weapon in addition to other weapons... but you're always allowed to fire an Ord weapon in 6th, as long as you move slow enough / not at all. It gives you special permission to do something you can do anyways... a result of edition change. To me, LB would need to be reworded along the lines of...
"Firing the turret weapon does not cause other weapons to fire as snap shots, and does not count towards the single weapon that may fire at full BS if you move."
That would make it clear that what I hope is the RAI behind LB becomes the RAW. I really want my LRBT with Plasma back!