Ah, yes. RedHat uses GRUB by default as it's boot loader. GRUB does not like USB keyboards. I cannot remember how, or indeed if you can work around this, so my advice would be to change over to LiLo. There should almost definitely be a package on the install discs for LiLo.
Oh, and before I forget. If you have some free space on your current harddrive, then go partition it and install your other Linuxen into them. If you don't, and you don't want to re-partition and re-install RedHat, then go get a/some new harddrive(s).
There are two main ways to multi-boot Linux. You can have one bootloader than loads them all, in which case you should not install any other bootloaders from any other distros. This is the more "elegant" solution, but can bring headaches if you constantly chop-and-change (as I do). In order to make this work, you'd need to install your Nth distro (sans bootloader); boot into your favoured distro (RedHat, for instance); mount your new distro's partition; copy over the kernel and initrd files; add them to your current bootloader; reboot.
Another method (and my prefered method) is to install your Nth distro, and install the bootloader to the root partition
of the Nth distro, and have an entry in your main bootloader pointing to that partition. That way you technically have multiple bootloaders, but it can save a lot of hassle.
For example, in my setup I am (currently) running Mandrake 9.1, Mandrake 10, SuSE 9 "technology preview" edition and QNX. The partition that Mandy 10 and QNX reside are constantly changing (I am a habitual tester and fiddler of distros). I have Mandy 9.1 installed on hda in it's entirety. Mandrake 10 is installed on /dev/hdb1; SuSE is installed on /dev/hdb2 and QNX is on /dev/hdb3. The entries in my lilo.conf file simply point to the partition, as follows:
When I select that, I am presented with another bootloader, this time from Mandy 10. It is by far less elegant, but it does allow me to chop-and-change my secondary/testing distros with relative ease.
Hope this helps.