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I have used Sunpak 522 "potato mashers" (w/sync cord) and Vivitar 285s ( w/hotshoe) without problems on the XPan I in normal 35mm mode.
In panoramic mode, getting a flash that will cover the wide angle can be real tricky. You will probably have to use something like a Stofen diffuser or use multiple flashes with remote triggers/slaves.
The Sunpak 622 has a bare bulb attachment that might also be useful in panoramic mode.
I've used the Leica SF 20 plus Sfill diffuser with success. I covered the Xpan's hot shoe except for the central contact with Scotch tape to avoid short circuiting, though. The Leica SF 20 is a very reliable and handy solution, although a bit costly... See pictures here.