How does Time Machine make room for new users? This is more of a hypothetical question, but we've been using a Time Capsule to back up our Macs, and it has steadily been filling up since the year or so that we've had it.

We've since brought a few new Macs onto the network and have started backing them up to the Time Capsule as well. From what I've read, Time Machine makes incremental backups of Macs, and apparently deletes old backups to make room for new ones.

My question is twofold: 


How does Time Machine decide which backup information to delete, especially amongst several Macs backed up to a single Time Capsule? 
As we add more users to the Time Capsule, and it begins to fill, will Time Machine be smart enough to give new users enough space by deleting old backups from other users? If not, how can we achieve this?
My experience is that TimeMachine does not cross the "per-machine" boundary when deleting old backups. 

We have a disk attached to an AirportExtreme AC and which is the target for four Macs. These machines have significant differences in the size of their backups. One of them in particular is an old Mini running 10.6.8 that does not see much activity other than the occasional software update, so the incremental backups are rather tiny. The MacBook Pros on the other hand are a lot more active and regularly submit incrementals that are several GB in size.

Whenever the MacBook Pros need space TM just discards their own history, never touching the others' directories.

This fact is immediately evident when comparing the number of backups for each machine. The Mini has a long history saved, the MBP's quite shorter ones.
Hope it helps.