I have recently bought a Canon 6D and want to use my Speedlite 430EZ with it. Is this possible, or are there compatibility issues?
For a Canon flash to be compatible with the hot shoe of EOS digital cameras, it needs to have a model number that includes EX. The EZ series of flashes made in the early 1990s are not compatible with Canon digital cameras. Some of the EZ series were auto only and have no way to be adjusted manually. Those won't even fire "dumb" when attached to the hot shoe of a digital camera.
In the case of your 430EZ, it can be controlled manually. If it won't fire manually when attached directly to the camera's hot shoe, you could use a set of simple non E-TTL triggers to transmit the "fire" signal from the camera's hot shoe to the flash. Some users have reported being able to get an EZ flash to fire on an EOS digital body by taping over all of the hot shoe contacts other than the large, central "fire" pin. You would have no automatic functionality whatsoever and would need to set flash power manually using the flash's controls and menus.