Adding New Computer to Already Full Airport Extreme

Hi everyone,

I currently have an iMac and two Macbook Airs backing up to a (now full) 2TB Airport Extreme Time Capsule Base Station. I now have a Mac Mini Server that I want to use with Time Machine as well, but I can't "turn on" Time Machine on the Mini because the target disk in the Extreme is full. Both the iMac and the Mini are hard wired ethernet connections. I've thought of at least two options, would like opinions:

1 - Get an external USB hard drive, plug it in to the Mini and use it exclusively for the Mini backups, keep the iMac and the Airs backing up to the Extreme. Probably the easiest to implement solution, since it maintains continuity with my current backup structure. But I don't expect the Mini to change much and it seems kind of a waste to have a dedicated drive to back something up that doesn't change.

2 - Same as above, but let the iMac back up through the Mini (either through the Server service, or just using the attached USB as a target disk). If I do this, I imagine Time Machine will delete the older and older iMac backups to make room for newer Air backups, or would I have to manually move/remove the iMac backups from the disk in the Extreme?

Thanks for your thoughts!