You can't boot linux from an external drive easily. But you can boot OS X from an external firewire drive. It is much much easier to put linux (I am using ubuntu) on the internal drive and boot OS X from the external, use something like carbon copy cloner
to mirror the OS X partition to the external drive and then install linux on the internal drive.
There is a work around for linux booting on an external drive here
for yellow dog and I know there is a similar one for ubuntu, not sure on gentoo but it should be the same. The disk access is faster from the external drive (I replied to Andy's post "mac mini addendum" with more info on external enclosures and USB Hubs).
The USB sockets on the keyboard are unpowered and will really only take the mouse even when I plug in a usb key OS X gives you a low power warning.