You can use a USB keyboard and a PS2 X-keys at the same time without any problems, but as you already know, you have to have a PS2 keyboard to program the X-keys buttons.

You do not need to leave the PS2 keyboard plugged into the X-keys after the X-keys has been programmed. The X-keys stores the keycodes in onboard non-volatile memory. However, if you're going to use a hardware broadcaster, you'll probably want to use a PS2 keyboard as I have not heard of any USB versions that perform as well as the PS2 broadcasters. Someone with first hand experience with a USB keyboard multiplexer can probably provide their opinions.