Can you chain a ps2/x-keys pro and an x-keys 20 key pad together and use a main usb keyboard for the system?
Yes. I have chained 5 X-keys together on a single PS/2 port and they all still work.
Oh also, are the x-keys modifiable at input time meaning if I press shift or control or alt on my main keyboard and press an x-key (say it is just a plain "a" for example), will the final input on the computer be an "A" if I'm pressing shift? I tested this with my n52 by making one button an "a" then pressed shift on my G15 keyboard while pressing the n52 button and it did modify the n52 input so I got an "A" instead.
I THINK so yes but have never tested it.

Does the ps/2 x-keys & pro work ok with with keyclone?
I don't see why not.

Does anyone know if the USB x-keys can be programmed with multi-lingual keys by using a ps/2 multi-lingual keyboard? I found one relatively cheap on-line and I like the idea of using keys I normally don't type to program into the x-keys units.
If you find out, let me know. You SHOULD be able to though. Not sure you can program a PS/2 with foreign keys though. I fiddled around with it around the time I stopped playing.