Five months ago we introduced puppy Mason to our household with the hope that nine year old siblings Molly and Murphy would just adopt him and love him and all would be joy. That didn't happen. It took Murphy a couple of weeks to get used to Mason, maybe he was waiting for Mason to get bigger, and then he started to play and they've been playing much of the time since. Murph still growls at Mason, especially if Mason interrupts a cuddle, but they get on; though I wouldn't say they were besties. Molly took much much longer to warm to the pup. Even after this long she still barely tolerates him and, for a while, took to marking her territory - which caused a lot of extra washing and some deep consternation from the adults in the house. We changed our routine - I now try to spend more alone time with Molly and she is happier. Mason just loves his big brother and sister. He follows Molly everywhere; when he isn't playing with Murphy. Mason is unperturbed by the growls and snaps and just gets on with being happy.
So, lessons? Theo has had his world turned upside-down and may be feeling left out. Try to make time for just him, be protective if you need as puppies have no social skills; these must be learnt. The lessons in good manners usually come with a bark and a snap and, sometimes, a bite. As long as no blood is drawn just let Theo put the puppy in his place. Continue to treat Theo as top dog, even if he ends up being submissive to Tiree. Don't hope for friends, just hope for cordial relations.
Looking back, Molly and Murph are not really friends, even after nine years; they don't play and largely ignore one another. Howver much we project, they are dogs, not people.