Go here -
http://www.saecollege.de/reference_material/index.html
click on "absorbers", then click on "co-efficient chart" - notice how big the difference in absorption of carpet is, depending on frequency? (This value only goes from 0 to 1, BTW)
Now, notice how much more even the absorption is with either vinyl, tile, or wood on concrete?
The goal in either a tracking or control room is to end up with as even a frequency response as you can get - in order to do this, you need to pick materials for surface treatment that will give as even absorption at all frequencies as you can get. If you put lots of carpet in a room, it sucks out the highs and leaves you with a dull, boomy room. By the time you compensate for this (if you even can) you will spend more time and money than necessary on low and mid frequency absorbers - Better to first find out what the room sounds like under known conditions, and only fix what's broken.
The main difference between a good tracking room and a good control room (other than size, and possibly shape) is the reverb time, known as RT60. The control room should have a shorter reverb time than the tracking room, so that you don't let the reverb of the control room convince you that you have enough reverb in the mix when, in actuality, it's coming from the control room.
Do a search here in the studio section for words like treatment, reverb, absorption, carpet, absorber - you'll find lots of comments that pertain to what you're trying to do - For that matter, the SAE site by itself is a gold mine of useful info. Check things out and post back, I think you'll change your mind about a lot of carpet... Steve
