Do you mean that you want to connect to your remote Host via Direct connection without adding them to your address book in case if they are stored on our servers? If this is the case, unfortunately, we do not store the remote connections credentials on our servers, i.e. each remote Host that you want to connect to needs to be added to your address book as a connection entry as described here. Sorry if this makes our software less useful for you.
What I mean is I use your servers for my clients due to bandwidth constraints. If I do that can I use an RU server for address book only so that multiple users can have the same access to all clients/hosts that are are on your servers? Make sense?
Yes, it's possible to set up any role for RU Server as a single role, so you can definitely set up the Address book sync only and still connect to your remote Hosts via our public servers. i.e. setting up the relay server role is not required for the sync role to work. Here's a related guide on setting up the address book sync role for the RU Server.