Yes, you can use a common data partition. If you need it to be accessible from XP too, your best choice would be ntfs. fat32 would work just as well - in fact, for a long time it was the standard for a shared partition considering that Linux could not write to ntfs in any reliable way - but now that we have the ntfs-3g driver, there really isn't any reason to use fat32 any longer.
Linux Installers have a partitioner so it is perfectly possible to create partitions during install. Personally, I like to make my partitions before installing because it gives a better idea just what you will end up with. Some file system types reserve space for metadata so that you end up with less than you partitioned. For example, if you create a 5GB ext3 partition, you'll have only 4.6GB of usable space. On top of that, some partitioners measure hard drive space in uncommon ways so that things get even more confusing. if you do the partitioning first, on the other hand, you can experiment all that you want until things are laid out exactly as you prefer. My preferred instrument is the gparted livecd, which can also reduce xp partitions. If XP is taking up all the space, that's the first thing you'll need to do, by the way.
No, you won't need to hide any partitions. As for filesystems, you can select those from the installer. Ext3 is a good choice, especially for smaller partitions.
In addition to your system partitions, you'll also need a swap partition (linux-swap file system). It is the equivalent of windows virtual memory. Just one for all three distros will do. Recommended size: twice the size of your RAM with a maximum of 2GB. If you have little RAM, swap is indispensable. If you have plenty (2GB and up), you may not need it at all unless you want to build a very busy server.
Some partitioners can do all the work for you but that is not recommended. It usually means that they'll take up all the available space, which is not what you want if you intend to install more distros. In fact, selecting the "auto" option may overwrite your XP partition too. Usual the manual option instead.
You can create as many partitions as you like but with limited space, you are better off with just one per distro. Select the partition and instruct the installer to mount it on "/" (=root). Make sure that you tell the installer to use the right one or you'll end up overwriting another distro.
5GB is not much, to be honest. If you use ext3, you'll have only 4.6GB of usable space and then you also need to make sure that you keep enough space on the partition free for temporary files. If you don't, you'll see performance degrading fast in no time at all. Debian will just fit in (about 2GB after installing) but you won't have a lot of room for additional software - and there is a lot to choose from. Fedora tends to take up a bit more so even for a basic install 5GB may be tight. If I were you, I'd install only two for now and assign them 7 or 8 GB each. That's just my 2 cents, of course.
Last edited by jay73; 11-05-2007 at 10:55 PM.