The short answer is "no." You can share certain partitions amongst distros. I use the same /boot, /home, /usr/src, /var/lib/mysql, /usr/local/SVN, and swap space between Gentoo, a Pure 64-bit CLFS, and a multilib CLFS. Sharing the /home partition isn't hard, but there are some gotchas that will getcha if you don't set things up correctly. From your current situation, you'd have to shrink your existing and create a new one in the resulting free space.
Originally Posted by creativenate88
suggestions of what distro ((or distros)) to add
Add all of them until you run out of room. Seriously, I suggest you stick with Ubuntu until you know you're way around *nix a little. Meanwhile, browse the (almost) inifinite number of threads here at LQ asking which distro is the best. That'll give you some idea what other distros offer. Then I highly recommend you start trying some other distros that interest you for one reason or another. That being said, I suggest Gentoo and (C)LFS.