Is the SunPak Pro622 flash unit compatible with the Canon digital Rebel XTI SLR camera?

Honestly all flash heads would work.

your real problem would be

1. the connections - if they are hot shoes or plugs etc.
2. you cant use the 3D matrix metering
3. you can't use +/- flash compensation

You can use A and Manual

Right after the shot you can decide on how the image looks and adjust from there. To be on the safe side establish your "standard" so that you would be guessing all the time.

Back to basic photography. Guide no. and distance

The SunPak Pro 622 does provide TTL capability with the Canon provided that you also purchase the correct module. However, it will probably only give you TTL capability, which is older, and not be compatible with Canon's newer E-TTL II.