I think you're right--the episode guide at TV Guide has Jay Harrington listed as a guest for eps 7 and 8...also, Rachael Harris, who played Shiela Sazz will guest in ep 8...and Leslie Hope,who played Anita Gibbs, will be back in eps 7 and 8. Paul Schulz, who plays Frank Gallo, will still be around too.
Good news, at least for me, the actors who portray Oliver and Nathan are not listed for eps 7 and 8.
But it looks like the pasts of several characters may, or may not, come back to haunt.
Watch Suits Episodes | Season 7 | TVGuide.com
ETA: Sarah Rafferty replied to someone on Twitter to expect Donna flashbacks, but she didn't say in what context.