The first thing you should do is setup localtalk networking between the laptop and the older powermac (since the iMac won't support serial based localtalk). You will need to do this since there isn't a prayer of fitting the distro files and installation partition on 50 MB.

I'm trying to figure out how much space an old install of NetBSD would take (you only need base and etc packages), but their ftp server is timing out on me. On a particular directory. Weird.