I've got a couple somewhat specific questions regarding dual booting between slackware and gentoo.

I don't want to format my HD, but I have an 8 GB windows partition that I'm thinking of trashing for gentoo. (I currently have slackware 10.1 installed).

I have a partition for /home. Would it be unsafe to mount this same partition as /home in gentoo, as well as slackware?

Can both distro's use the same swap space?

I am assuming that this will not cause any problems since the distro's cannot try to access the partitions at the same time.

After reading other threads with similar topics, it seems that making a /boot partition is helpful because you can edit grub/lilo from either distro. But if I delete the partition that is at the first sector of my drive, aren't I unable to create another partition in its place? Would it be possible to get around grub and install lilo instead? and just install that to the MBR?

Sorry if I put too many questions in one thread.

Different distro's can use the same swap space but you may run into problems tying to use the same home partition. To get it to work you'll need to make sure that you have the same username and UID number on both of them, otherwise you may find that you don't own your home folder and can't login

I know that I used the default UID when creating my main user in slackware, so can I safely assume that if I use the defaults while making the user in gentoo, I shouldn't have any problems ... ?

I share my /home between Slack & Gentoo, it's no problem. Very occasional error messages are about all.

Sharing swap space is zero problem. I also share /boot between all distros to make Grub easier to use.

A separate /boot is a very good idea - you can delete a distro, and still have a boot-able machine.
Truly useful if you are playing with multiple distros.
Don't understand the question re the first partition - you can always replace a deleted partition. No partition is in the first sector - the MBR (and partition table) is.
You can replace the boot-loader at any time; I'd suggest you pick one, and stick with it. Subsequent distro installs should not install a boot-loader; use the one you have already on the disk, and update the conf to reference the new distro.