Aquactinics makes their halide fixtures such that you are able to set them on top of the aquarium without increasing the temp of the tank. I think many of the shallow water corals can stand the amounts of UV and IR that halides put out, as the sun does put out more. But yes, anything other than corals from the shallows, and they are likely going to fry with running your lights very close.
I'm not running halides anymore..... When I did, I shot for 10-12 on shielded DEs. With lumenarcs and SEs, a little closer, say 8".
With T5s, initially, I ran them almost on top of the tank (PowerModules, SolarFlares, retros). But not, since I miss the top down, I have raised them a good 4" off the top of the tanks.