Sovan,
You will be able to add published services from 10.6.1 ArcGIS Server as items to the 10.7 Portal.
Federating that server to the 10.7 Portal would not work as the versions of both would need to be the same.
Hope that helps!
-Danny
EDIT: Federating the server will work, but you will not be able to set it to a specific role, such as Hosting, Geoanalytics or Raster Analytics