I Can SSH host computer but computer does not appear in Finder's Shared Computers List?
Bonjour is the protocol that is used to "discover" other computers and will list them in Finder.  Bonjour only works on the local network.


  I want to connect via Apple's ScreenSharing.app , to my home mac from
  a separate network.


If it's on a separate network, you won't see them in Finder even though you can still reach them (ssh, scp, ftp, http, etc.).