I have an old street light that has a multi tap ballast, the only markings that I can read on the wires are 240volt. My system is 3phase 208 volt and I was told that I could get a 2 pole breaker and the fixture would work. Is this the case?

Yes it will work, cannot say how well it will work at the lower voltage. If the ballast is a multi-tap, one tap is for 208 volts.

With some testing of the ballast windngs with an ohm meter, you should be able to figure out which tap is which.

I have never had to do that, so easy for me to say, as I would be sure the ballast had the proper markings, or sell a new ballast to the customer.

Again, easy for me to say, but when I do a job for a customer, I need to warranty it for a year, and I do not need any headaches with return trips, lights not working, and the possible liability from either.

For a single phase 208 volt circuit, yes. Only a two pole circuit breaker is needed in a three phase 120/208 volt panel.