I think the proper order is Panther first, then ubuntu although I haven't really got experience on how OS X behaves when there's another OS on the drive. On the other hand I know well how a certain one from Redmond behaves... At least I'm fairly confident ubuntu will behave well so I'd install it after Panther.
For partitioning, I'd suggest at least 5GB for Ubuntu although you can get along with a bit less (typing this on a machine with 3GB HD but it's pretty full). In any case, you can also use your OS X partitions from Ubuntu so you don't necessarily need much space for anything else except the apps.
You can just have one big root partition and a swap partition. /home can be nice to have on a separate partition as then you can easily do reinstalls but more than that isn't really essential but I do tend to have separate partitions for /usr, /tmp, /var if the disk I'm installing on has the space.
Getting OS X to boot after that should not be a big issue. If Ubuntu doesn't handle it itself, you can just add a "macosx = /dev/hdX" line to your /etc/yaboot.conf and run ybin. After that you can select which OS you want to boot into.