You have to pay for any domain you use.
As for nameservers, if you set it up properly with your main domain, any 'addon' domains can just point to your nameservers (n1.domain.com, n2.domain.com) If you have only 1 IP, there is a tutorial in our Docs: HERE
As for nameservers, if you set it up properly with your main domain, any 'addon' domains can just point to your nameservers (n1.domain.com, n2.domain.com) If you have only 1 IP, there is a tutorial in our Docs: HERE