Since you have a legal XP license for the desktop (even though it isn't installed on a HD) and licenses for the Vista laptops you are entitled to use the triple user upgrade version. Upgrading from XP requires a full install anyway even though it is an upgrade (without the XP on the drive it may require you to boot up with the install W7 DVD twice - I've never heard of it asking for the XP disk but have it available). The truth is it is also recommended to do full installs on the Vista machines (from the upgrade disks), and depending on which version of Vista you have it may be required (for instance if your Vista is 32-bit and you want to install 64-bit then it's a complete install, or if you have Vista Ultimate and are "down-versioning" to home premium it's a complete install).
As for needing Pro, the Pro version does offer two extras I find useful, one being the group policy editor and the other being free use of Virtual PC - XP mode (though this second one is not as important for you because you have XP disks so you could use
Oracle's VirtualBox for free). So if you don't do many registry edits that would require the group policy editor then I think Home Premium may work fine for you. So if Home Premium is all you need then the triple pack should meet all your needs; you won't need the Pro as well.
W7 can see drives up to 2TB (4 x 500GB) so you shouldn't need a floppy.
If you do decide to do the full install over Vista rather than upgrading I recommend you download the Vista to W7
WET (Windows Easy Transfer) tool. Use WET to back-up you userids and data (hit the advanced option and verify it has all the data you want) to an external drive or DVD. With WET you would install W7, then restore with W7 WET which is built-in to get your userids/data back, then you reinstall all your software.
NOTE: WET will not work going from Vista 64-bit to W7 32-bit; all other migrations (32->32, 32->64, 64->64) are supported.
One more suggestion. Even though you are only upgrading the laptops from Vista I still recommend running the
W7 upgrade advisor with all your devices (printers, projectors, external drives etc) plugged in to allow it to look for program and device issues.