Thank you SO much for adopting this older fellow. I wish I could offer advice, I can't. Yoda is an older wiser fellow, I'm sure he'll teach Mason what manners he may be lacking in appropriate "how to treat a senior" area, though. Be patient with both of them. There'll surely be some growls and snaps. Perhaps keep them separated a bit, let them get the scent of each other first? Again, no real advice and I imagine there's lots of folks on here who have been in your situation and give you some good advice.
Neutering isn't as invasive as spaying, so honestly, I think Mason will be alright staying on the schedule you've got.
1. Ask the people to send 2 blankets/tshirts/large rags or towels along with him that they have used with him NOT WASHED. So he can have their scent. I do this with pups and rescues.
2. Get a martingale collar as several people have had a dog spook and never find them again....just be aware it can happen and stop only if you need to but with the martingale he can't get loose.
3.Does he(or is he used to a crate) are they sending his along if he is? This will provide comfort if he is used to it also.
4.As suggested if you can meet a short distance from your home and walk the 3 together this would be great.
5.Have gates as suggested as you do not want to leave them alone if you are gone and also to geive them breaks so you can give them attention seperate and play with them together. I would not bring out toys till you see how they get along. Feed seperately also.
6. Honestly I have never had a real problem with rescues but this is how I have done it and mine have mainly come from puppy mills.
7.Expect at least 30 days for him to get used to everything but then again he could just adjust very easily. Do keep an eye on them till you know for sure!
Thank you for helping out a senior...I am sure he will do well and love you in no time!
Congrats on adopting an older Corgi, all my rescues have been older Corgi's and I would not have traded any of them for a younger dog. I intoduce the new dogs pretty much the same as Jane does and have also never had any problems with it. Just make sure Yoda understands you are the leader right from the start and don't feel sorry for him loosing his old home it will just make him try to take over.
I would go ahead with Mason's neutering, as Ellen said it is not invasive and recover very quickly, and hopefully Mason won't be so full of himself when Yoda arrives since he will be recovering from the surgery, and Yoda may be more accepting of Mason if he is not meeting an "intact" male. Just expect a few disagreements between them because Yoda WILL teach Mason how to respect an older dog. Follow Jane's suggestions and you will have a good outcome, he will miss his old family but he will get over it in a short time, and you will have an awesome old man to enjoy!
Definitely have a safe area where the new guy can be away from the puppy. A ten-year-old may put a pup in his place, but then again he may not. Sometimes older dogs just stand there and look miserable while pups crawl all over them. My parents got a puppy when their old lab was around 10 and he let her steal his toys, steal his bed, climb on him etc and would never correct her. My parents needed to intervene sometimes or the pup would just make the lab miserable, and it did contribute to that pup growing up thinking she was the queen of the castle.
Treat the new guy like a pup as far as house breaking. Meet in a neutral spot, walk them together, after a nice long walk THEN let them sniff and greet. Take them both home, lock up your puppy and take the new guy on a tour of the house. Let him explore, any signs of leg-lifting give one firm "no" and a gentle but sharp leash-tug.
Depending on how they interacted outside, you might then let them both loose in the house or more likely put the new guy in his safe place and let them get used to the idea of being in the same house without interacting. A little later you can let them loose together.
For the first month or so I would not leave them loose together when no one is there.
And the new guy should not be left loose unobserved. Like I said, treat him like a puppy: if you are not right there to watch, put him in a crate or pen. Take him out ever couple hours. Let him earn his freedom. Even perfectly house-trained dogs sometimes get confused in a new house; some dogs narrowly interpret house-breaking to mean "I must not potty inside the house I was raised in" and will potty inside in new places. And then the stress of meeting a new family and dog can cause accidents and/or marking.
Err on the side of caution, and let things develop slowly in their own time. Good luck!