the general agreement on how much live rock is aprox 1 pound of rock to 1 gal of water.
It's a nice start, and lets you add more later as you aquascape your tank, or that beautifull mushroom rock just HAS to come home with you.
In my 150, I have well over 265 pounds of live rock (I just added more! lol). It also has 5 Inches of sand (I am one of those deep sand bed people) skimmer, wet/dry and tons of water movement.
I have found that the rock does an amazing job of filtration, and the deep sand bed is wonderfull when stocked properly. I have aprox 22 nassarius snails, sand sifting gobys, a red linkia, and whatever else decided to live in the sand. Oh, my hermits clean the sand up pretty darn well too!
Now, as to what Thales mentioned about the deep sand bed.. that is ABSOLUTLY correct. You can't just leave it. I Vac it twice a month. And if I see any build up, or discoloration underneath (yes... I check under the tank too! lol), then I go ahead and target that area for extra cleaning.
Ok, sorry.. I digressed... I love live rock.
It lets me keep more critters than I probably should.. or at least it HELPS in letting me keep more than I should.
But it is only one part of the entire filtration mechanism.
Each part is important to the entire machine working at its greatest potential.
Oh, and I think you are concerned for the organisms that are already on the rock in terms of temp. I really wouldn't be too concerned. They will addapt to the temp you decide to keep, and if not, will be replaced by others that do not have a prob.
Hope this helps.
Cheers,
Geoffrey