You can run Match on her computer, provided she does NOT
- run Match herself
- re-download purchases
- use autodownload for purchases made elsewhere
Those things can only be done for one AppleID per computer (NOT per user account) and that can only change every 90 days. That is, if you run Match, you will lock her out of doing any of those things for three months.
Streaming music through Match does down use any disk space; you will only use disk space if you download music.
In principle, you can "merge" your libraries, in the sense that you will log in with your AppleID, see your Match cloud content, but still be able to see and play the stuff that is in her library on disk (might ask you to authorize it again, I'm not sure).
Hope this helps
No, that's not true. You can access the store to buy new stuff or authorize existing content. You only need to "associate" the computer to re-download, auto-download, or Match stuff (see association rules:
My kids use their iTunes store accounts on the same same computer on which I run Match with no problem. I did, however, get locked out when one of my sons *re*-downloaded something he bought on another computer.