I've had a rottie before and he was nasty to the other dogs. It was just his personality. We had to rehome him to a family with no children or dogs.
Not placing any judgment on you, but is it a wise idea to take in another
puppy right now? Again, I don't know you or your situation, but I would think very carefully about adding another right now.
How long have you had Thor for? How is his training progressing? If he is not reliable yet, I would restrict interactions between the males right now. They are right at the age where they start clawing for leadership. I would do msvette's 2 week shutdown and limit interaction with the new dog. 2-3 months is not very long. I would keep it separate and not invite any trouble by trying to get them to be friends. if I were you, I would just wait out the 3 months. 3 months is not long at all and no reason to have long term issues over this short stay. You are going above and beyond for this family. They should also help you by giving the dog daily walks, trips to the park, hiking, socialization etc. I would recommend them to take an obedience class during this time to keep up with training and to give you a break.
Is this dog intact? Does he have any aggression against dogs/children/etc? That will invite further issues if he is...
Take them on a low stress walk with a separate handler for each dog. Don't force any interaction. Let be casual and on neutral ground. Do not let them meet in your house or yard. Take them to a park, take a quiet walk and don't allow any off-leash interaction yet. No toys, treats or anything of value till you can trust them 100%. Even seemingly stupid things can cause fights. Puddi and Whiskey once got into a snarling match over who gets a piece of a chewed up dish towel that one of them found under the couch