4. Can I use one TM disk to back up 2 or more Macs?
Yes, if it's big enough (see question #1). TM keeps track of which computer is which (by a hardware identifier, not computer name), so it will always keep it's backups for each Mac separate from each other.
It is advisable, though, to partition an external disk into one part ("Volume") for each Mac (see question #5 or #6 for instructions). Unfortunately, you can't partition a Time Capsule's disk.
There are two reasons for this:
First, TM will, eventually, fill all the space available to it before it begins deleting old backups. When multiple Macs are "competing" for the same backup space, TM might have room for several months of backups for one, but only a few weeks for another. This is especially likely to happen if you start backing-up a new Mac to a drive that already has a lot of backups from another Mac. TM on the new Mac will not delete backups from the other Mac, so when it needs space for new backups, it will delete the oldest backups from the new Mac instead.
Second, if you ever want or have to delete all the old backups for one Mac and start over, you can just erase the partition via Disk Utility; if there are other Mac's backups there, they would be erased also. If you replace one of the Macs, TM on the new one won't delete the backups from the old one. And sometimes it's advisable after certain repairs or problems.
It is possible to delete individual backups via the TM interface, but it's one-at-a-time, so rather tedious. See question #12 for details. (And don't be tempted to just delete them via the Finder -- all sorts of things may go very wrong.)
