I think you are mixing using "vanilla" (the kernel from Apple) with installing "retail" (the original retail DVD instead of a distro) so I'm not sure what you are trying to achieve. As for the Quad Boot I think you are trying to do something too complicated without having the ideas really clear. You should have the concepts really clear for doing that and plan how are you going to install the OS.

You can install the Retail Leopard on a GPT or MBR (with a modded installer) partitioned drive. Any method you could use will NOT install any bootloader, you have to do it on your own, so you should have the choice of using PCEFI or whatever. I would not go with anything apart from Chameleon 2 right now but that will not let you boot Snow Leopard from what I know.

If you have a GPT style partition, you cant install Leopard on the EFI (System Partition), you have to install it in a primary (GPT) partition; you can however boot Leopard from EFI System Partition (thats munky method). Not sure if its really recommended, I think Windows 7 would like to boot from EFI System Partition by default but maybe it will boot from a primary too not sure.
In any case I think Windows XP will NOT install or boot from a GPT disc. Ubuntu got troubles booting from GPT on non-EFI system I think, but probably they fixed them already.

If I were you I would try this route:

- Install Windows 7 on a clean disk, making sure you leave enough partitions for the rest of the OSes.
- Install Retail Leopard on a primary and chameleon on the same place, should overwrite MBR and you'll have Chameleon as the main bootmanager.
- Install Ubuntu on another primary making sure GRUB goes only on the partition, NOT on the drive's MBR.

I havent tried that so I can assure how it will go.