Scotland head coach Gregor Townsend has made three changes to his starting XV for Saturday's Guinness Six Nations trip to Italy.
Hooker McInally swaps places with Glasgow Warriors' Fraser Brown, who moves to the bench, while second row Toolis comes in for the injured Jonny Gray.
Harris has been selected ahead of Huw Jones, who drops out of the 23-man squad. Sale wing Byron McGuigan takes Harris' place on the bench.
Speaking about the changes, Townsend said: "We believe Chris and Stuart deserve this opportunity. They were very close to the starting 15 when we entered this campaign. We think this is the right time for them to come in.
"Chris has been excellent for Gloucester this season and has continued to make improvements since the summer. He works very hard on both sides of the ball, which will be important this week.
"Stuart has shown over the past few weeks he's back to full fitness and is raring to go and Ben gets an opportunity to start following Jonny's injury against England.
"Second row is a position (where) we have a lot of competition with a number of very good players available for selection. We're looking forward to seeing how he combines again with Scott (Cummings) after a successful outing together against Russia in the World Cup."
Scotland and Italy have both yet to win a match in this season's Six Nations, Scotland losing to Ireland and England and Italy suffering away defeats to Wales and France.
"Italy are always a very tough opponent, especially in Rome, where we know they'll look to play with a huge amount of energy," Townsend said.
"It's a different Italy team to the one we've played in the past. In their opening two games of the Championship they have played with real ambition and width.
"They've also selected athletic ball players who suit this strategy and are comfortable playing at a high tempo. We are well aware of the threats throughout their team and how their attacking game can cause problems for any defence."