Russell does everything fairly well - pretty good IQ, skates well with the puck, moves it well, fearless shot-blocker and he has a surprising amount of grit in his game.
The only knock on him is his inability to contain large forwards along the boards. That is my single reservation with him - and I think it is an important one.
Now, put him on a team with an oversized blueline, and he will compliment that team exceedingly well I think. I just don't see a fit for him in Calgary. Engelland is the only physical defencemen on the Flames (and I actually expect him to be re-upped sometime this season), with Smid possibly never playing again. Then you look at the western conference, and you see a number of teams that have quite large and physically imposing forwards - even Edmonton got a lot bigger.
On a team with some larger top 4 D - I would have no issues with rolling out Russell as a top 4 D.
On a team without size and grit in their top 4 - I think Russell needs to be sheltered a bit and belongs on the third pairing.
In Calgary's case, Engelland is a 'must' (imo) to play as he is decent defensively and offers quite a bit of grit. Wideman (or Jokipakka - depending on who lines up with Hamilton this year) gets the spot next to Engelland. You then have cheap options with Wotherspoon and Kulak coming up - and depending on who you are reading and listening to - Kylington may get a good look up on the big club as well (wouldn't surprise me if he gets some games in, but would surprise me if it was a really long look).
Signing Russell at anything over 2.5 for a third pairing isn't the best use of cap space. For that reason, I wish the Flames don't sign him, though I like him as a player. Different story if the Flames had a more physical top 4, then he wouldn't need sheltering (imo, again).