Accessing MS Proxy Server accross domain

I have 4 NT domains, with Proxy servers in 1 domain. What needs to be done to allow the other 3 domains to access the proxy server? I am thinking in terms of authentication, etc.

Also, if I add another proxy server to one of the NT domains, is it possible to allow 1 proxy server to act as a fail-over?

Thanks,
Tom Morris
Network Analyst