Yes, iTunes Match associates your computer to one AppleID, and this can only change once every 90 days. No one else will be able to download previous purchases or run Match on that computer for 90 days after you start Match, and if they do after that, it will lockyou out for 90 days.
In contrast to their authorization rules, which are pretty transparent to legitimate customers, this is an incredibly annoying rule that punishes loyal customers and does nothing to prevent illegal sharing. I encourage everyone to write the nastiest feedback possible.
In answer to your other questions:
No, Match will not match songs authorized to other users (I think it informs you about any, but I can't remember).
Yes, once you have Matched a computer, you can deauthorize it, and even deassociate it and you will still be able to see your Matched music on other Matched devices. However, even doing both of these things does NOT change the 90 day rule.