When you aim the fire template, are there limitations, like not having the template over a unit that's in CC, or having it touch some of your own units?
Yeah you cannot deliberately hit units in combat or your own units. Of course there is a bit of a loophole there if there is another unit in between, you choose to target them, and it overshoots into the unit in combat, that is allowed. Honestly, I found the thought of that a bit beardy so I pretty much don't point the salamander where there is a chance it will end up on a unit in combat. The thing is powerful enough as is.
I fully agree with Strewart on the loophole issue - it is definitely not sporting to allow an "overshoot" to hit units in combat. With being able to measure all distances in 8th ed, I find this is easy to avoid simply by measuring out to 18" to ensure there is a clear line of fire with viable targets.