Not sure about the benefits of running ECLRT and Finch as a single line, even if the train length is the same. The single-line operation will make it more difficult to run smoothly and avoid delays. And the number of riders who want to, say, board at Eglinton & Kipling and travel to Rexdale via the airport will be fairly small.
The non-revenue connection between the lines will be very useful, for sure.
Speaking of revenue service, it will be more sensible to add a north-south line, that starts at the Subway Line 2's Kipling station and partly shares tracks with the ECLRT and Finch lines. For example, using the Hydro corridor to Renforth interchange, then up Renforth and Silver Dart to the point where the extension of FWLRT connects, and then up Hwy 27 to Humber College. Or, just straight up Kipling, although in that case there will be no track sharing.