Hi RK,
yes. The relation between databases and RepServer instances is many to one. A database must be controlled by exactly one Replication Server (regrdless of its role as primary or replicate - I guess that's what you mean as "secondary"). A single RepServer can control multiple databases. Just keep in mind that SAP recommends connections between DB servers and RepServers (in either direction) to be LAN connections. If there's WAN between primary and replicate DB, this part should be covered by a route / a RepServer-to-RepServer connection.
Out of own experience, I generally recommend to take thr first steps on RepServer with somebody (maybe a consultant) onsite who is not doing this for the first time.
As for the RSSD / ERSSD question: I agree with Mark's answer if ASE is involved anywhere. Recently, there have been quite some RepServer deployments w/out ASE involved (mostly from a 3rd party system to HANA) where nobody will run an ASE just for RSSD. And even with ASE part of the scenario, the ERSSD is usually on the RepServer machine, consuming few resources and reducing complexity. I think if you feel comfortable with one or the other option, there is no strong reason not to use it.
HTH
Volker