We are planning to use Portal for ArcGIS with ArcGIS Server in a non-federated/loosely coupled environment.
Can anyone please confirm if we can use Portal for ArcGIS 10.7 (to be released in spring 2019) with ArcGIS Server 10.6.1 or both Portal for ArcGIS and ArcGIS Server must be in same version? We will use map service, feature service, gp service and geodata services only.
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!
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
Please check Portal compatibility with earlier versions of ArcGIS—Portal for ArcGIS (10.6) | ArcGIS Enterprise although 10.7 release of documentation will be available when ArcGIS 10.7 is released (expected by end of March 2019).