Hello!
I think that you may use them "in parallel topology" to make sure that you will get internet even if one of the firewalls will fall down.
If you have high DDoS rate attacks maybe you need specialized anti-DDoS device. But it will be needed if the USG will not be able to stop the attacks. If this will happened, I think that you will need to consult with "real" engineer to check you problems and find the solution.