A run of fine form fr Glasgow late last season saw Cummings force his way into the Scotland squad for Japan where he played some part in all four games.
Speaking ahead on the trip to face Zebre in Parma, Rennie told the Courier. "Even last year when we got to the quarter-finals in Europe, we left Jonny Gray out in favour of Scott, who had been outstanding while Jonny had been away on international duty.
"He's a smart kid. He does a really good job around running the lineout, and he's a real point of difference from most Scottish locks, I think, because he's an athlete and picks good lines to run, and he creates good go-forward."
Rennie also believes Cummings has added extra physicality and aggression to his game.
"He's developed really well over the last 12 months around clean and tackle, and got a lot more brutality," he continued. "When Jonny forced his way back into the team, it was alongside Scott.
"I think Scotty was one of our best in the play-off series, and one of the best players on the field in the final.
"It was great to see him get an opportunity at the World Cup and kick on and be invaluable to them."
"We always said we were keen to play Tommy at 15 and in Cardiff last year he played really well there," said Rennie. "He is good under the high ball, has a great skills set and is experienced and can organise the back three. We have pretty good depth at wing so we're keen to see him there.
"We spoke to him about it last year when we signed him on longer term. Going forward that will be a good position for him and he was pretty sound about that."
Seymour's move from wing will also open the door for Ratu Tagive to play more, in addition to the wing now being a Scottish qualified player.
"To be honest he has really forced our hands this year because he is playing superbly," said Rennie. "Every time we have given him an opportunity to play in pre-season he has been excellent.