Not sure about event ships (my engineers are low level), but for premium ships, and
engineer in the BO slot still adds to overheat.

you can use any at level sailor for the bridge operator slot, however an engineer that
isnt in the support slots will not contribute to the engine speed

Therefore it would be beneficial (if you had no BO ONLY) to put a repair sailor as the BO
because his stat will still work regardless of where he is located on the ship.

..or so I was told

I just Checked with my Mikasa EBB

I can put any BO onto the Bridge (never knew this!)

I can put any Support Sailor (I tried Engine and Repair) onto the Bridge.
The Engine Does not Work on the Mikasa EBB, (Repair not sure)
I believe it will work on Premium Ships as a BO and an Engine.

6 Support slots on haruna....

Not 100% sure about this, but I think supports as BO only works for prem ships, not
event ships.

even with eng as bo?
hear eng give longer lines.

6, yes, yes with short lines though

6, yes, and I'm not sure, respectively.