Can I have more than one domain on the same Rcs.is?
There are no limits as to the amount of domains you can point at a Rcs.is and you can serve multiple websites from a single Rcs.is.
If you are using Apache to serve websites on your Rcs.is, you will want to set up your Apache virtual hosts to host multiple websites.
If you are using nginx to serve websites on your Rcs.is, you will want to set up nginx server blocks to host multiple websites.