All great questions!!! First off let me say, all dogs are individuals so what might hold true for one dog may not hold true for another. You will be best served by knowing your dog and yourself (i.e., how much time you have to dedicated to training and being with your dogs, what you are comfortable handling in regards to dog behavior and pack interactions, etc.)
What I can tell you from my personal experience is that I have been beyond blessed to have spent four years raising and training my first girl, Lila. So that when I brought a second dog home, a then 5 mos old Zeek, Lila and I were tight. We knew one another. We knew what each could handle. We had super solid training and communication skills. So when Zeek came into our house, Lila was able to help me raise him. She helped to support my teaching him the rules of the house and what behavior was expected. She helped to support and give him confidence as he went through his last few fear stages and keep in in line when he went through his slightly obnoxious adolescent stage. When Zeek was four and Lila was eight, we had a super tight pack. So two years ago when I brought home my, then 4 mos old Pru, both Zeek and Lila are able to help me raise her. When I brought home my next pup, the first pups were solid and required less training time - allowing me to focus more time on the new pup.
This help and support from my adult dogs has been PRICELESS!!! Pruie is a spitfire who will push the boundaries anytime she can. Zeek and Lila have the patience and wisdom to know when to let it go and when to make her tow the line. (Granted this is primarily my responsibility, but let's face it, they're dogs. They speak her language and they know her energy better than I do and if I don't see a potential problem, they do and they either alert me or address it themselves. Having four and eight years of experience with them, I know that I can trust their judgement and know that they will handle her appropriately. Again, this is INVALUABLE!
To address your specific questions:
1 and 2) It will all depend on the individual dog. Lila (Great Dane mix) would be fine and adjust to almost any dog I brought in the house as long as they were stable and appropriate. But I did not want anyone challenging her or pushing her so my next dog was a young male. These two fit together like peas in a pod. There has been absolutely no challenging of Lila. As a matter of fact Zeek has always been, "Yes, big sister." "No, big sister." "My apologies, big sister."
Zeek (EM) on the other hand is very particular about his dog friends so after some meet and greets with potential additions, we quickly learned that Zeek was going to be most successful with a puppy - someone who was not going to threaten or challenge him and any rude behavior would be forgiven under the "puppy pass."
Enter Pru (AmStaff). As a six month old puppy, she tried her hand at challenging Lila. For me, that behavior would have been a deal breaker for Pru continuing to be a part of our pack so I shut it down (Lila addressed it, but Pru pushed anyway.) Zeek supported myself and Lila in a way that Pru got the message. She tried it once and the no more. If Pru had been an adolescent or an adult dog, it may have taken more time and management to remove that behavior as an option and we might not have been able to remove it entirely. So for me, I would say the combination of age and gender are important factors.
Also remember all dogs are going to need individual training and bonding time. Will you be able to give each dog the time they need for bonding and training?
3) Having a 5 month old puppy, he is just beginning to mature sexually so I don't know that you or he really know who he will become once adolescence and adulthood hit. This is where the relationship between the two dogs, your confidence, management, and your relationship and training with you resident dog are going to be critical. If I were in your shoes and had to bring a second dog in right now - just to hedge my bets - I would either go with a female puppy (like 12 weeks) or a female adult. If I were going with the adult female, I would want my 5 month old to naturally look to her for guidance. (Yes, big sister. No, big sister. I'm very sorry, big sister.) This of course also means I want my adult to be appropriate and have good manners to help teach those to my puppy.
4 and 5) I, personally, am not a follower of a two week shut down (something you see a lot of shelters and rescues advocate). Instead, I watch and manage. When I can't I separate. Each of my dogs are kenneled until they prove they can be trusted when unsupervised. So, for me, when the new puppy came in the resident dogs had already earned free roam. For me, SO MUCH BETTER than having three kennels! I will also say that when I brought the new dog home, I also took time off from work so I could start the bonding and training process. Usually my process goes something like, morning walk for all dogs, out together for a new hours - with me 100% focused on the dogs. Everyone takes a break and puppy goes into his or her kennel for a nap. Out again in the afternoon play, more walking, and just hanging - again 100% focused on the dogs and if I can't be, then puppy is kenneled. Puppy sleeps in kennel in the evening. This has been successful for me, but I will say that it is EXHAUSTING!!! The last time I did this was almost two years ago and I still can feel the exhaustion!
Again, only you know you what want to and can take on, but if someone were asking me, I would say wait on the second dog. Get your first dog through adolescence - see who he becomes. If you want them to have friends and companionship, set up some play dates with your friends and neighbors and/or find a trusted daycare. These two things will also give you more information on your dog and what they do and do not enjoy in their dog friends.
Another thing to keep in mind is if you bring home a second dog of similar age and breed, God willing A LONG TIME FROM now, they will age and eventually pass. Can you and do you want to be handling two aging dogs simultaneously? Potentially losing to dogs back to back? Beyond heartbreaking, it could also be very expensive. (Just more food for thought.)
