Trigger voltages of old flashguns - will they damage my 600D?

[TotoEC]

May be a way to use the old flash is to set it on an optical/wireless trigger and fire it in manual mode.

Yep. My 1980 Sunpak optical trigger still works to-date and I'm using it with the Vivitar 285 which I'm using as a background or highlight flash in manual mode. 

[zim]

After you've used it for a while please post back and let us know how the camera is. Everything I read when I had the same question came out on the side of 'don't do it' so I'd be real happy to be able to use my old but powerful sunpak 

[sama]

I, too, thank you, Mt. Spokane.  And raise a virtual Sam Adams (or Yeungling).  Salut!

or do you mean "Yuengling" ?

[Gothmoth]

while newer canons may accept up to 250v keep in mind that some flash have 350v or even more. up to 600v i have seen.