I have several Sigma lens that I use on my Canon 20D. One of them is the 70-200 F2.8 which is a huge lens. I've never had any trouble mounting it on the camera. My other lens include several Canon lens and one Tamron lens. All of them mount with equal ease. Can you be more specific about the problem you have in mounting it?
There is no reason that the lens would harm the camera if it was left mounted all the time.
Cheers,
rfs
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