Basically, your going to want to make your entities compliment the design of the room. If it's a large room, add quite a few props but spread them out a bit. (The trick is to not have many huge open spaces in the first place, that hurts your Framerate as well

)
Honestly the service room doesn't look bad at all! Maybe add some crates or barrels or something to fill up those empty spaces. (Model Pack 3 can definitely help with filling empty rooms but u can make stock work just as well

). Keep up the good work dude
One more thing, i agree with Dar13, remove the pipes in the last shot or at least get them off the floor, those will be a pain in the ass to move over
Yeah... got hurt when i tripped... on some bullets