They are not individual units, it's 1 unit. But can have mixture of weaponry. So can the Shootas shoot at something, the sluggas at another, and Big Shootas at another.
I would think they all have to pick the same target but just in case.
The footprints made in the sands of time are not made by sitting down... Unless your playing Warhammer WAAAGH!
New times calls for new Records:
Orcs & goblins (3-1-2)
Imperial Guard (0-0)