How to safely introduce an aggressive dog to a new puppy at home?

Walking them in neutral areas away from his normal routine and territory can help to get dogs used to each other in a less confrontational manner and can help in such cases, I would recommend muzzling both before allowing free play to prevent injury initially. Do not leave them unsupervised or loose until they are more accepting of each other

You're right that there can sometimes be problems when introducing two dogs for the first time, especially if either of them has any history of aggression towards other dogs. I would recommend introducing them in a neutral place at first, like a park. Keep both dogs on leash, let them sniff each other briefly, then go for a walk together. This allows the dogs to get used to each other's presence without feeling pressured to interact. After the walk, take them both home and into the house together and if things are going well, let them off-leash to relax. If things don't go smoothly, it would be best to get help from a veterinary behaviorist (ideally) or a good reward-based trainer who can work with you in person. You can search for a behaviorist in your area here: www.dacvb.org or a qualified trainer here: www.ccpdt.org.

Any introduction should be done gradually and on "neutral" ground. You can use a pheromone product such as an Adaptil collar to decrease anxiety or aggression. Introductions should start with Max being able to hear and smell a new dog but not see him. This can be done a friend's house or perhaps a family member's home. Again, some place neutral to both dogs. Once he is acclimated, he should be able to see the new dog but have no physical contact. This can be done using the fence at a dog park. Eventually, you will be able to physically introduce Max to the dog. Use a leash on both dogs for safety and muzzles if necessary. Go slowly, don't force him but correct any negative or aggressive behaviors immediately. Praise and reward him as he progresses.

I personally would not recommend getting another dog for the house just yet. I'm quite concerned that Crosby is aggressive at the dog park and when you went to see a puppy. I would be very worried that he would attack a new puppy, causing it serious harm. The best thing to do is get him into a professional behavioral expert to work on his aggression. I never recommend owners try to deal with aggressive dogs on their own due to safety risks, it's best left to a professional. The professional can work with him and you on his aggression issues, and they can give their opinion as to whether or not another puppy would be safe in your home. Unfortunately some dogs are meant to be only dogs due to these issues, but certainly work with a trainer first before coming to that conclusion. You can ask your vet for a recommendation to one. I hope this helps!

If your dog does not get along with other dogs it was probably a very bad idea (and a very stressful one) to bring a new puppy into the house. A new puppy can also be hard around a newborn as they aren't yet sure how to interact with each other. You can try to continue to get the two to get along by starting with limited introductions that are supervised, however if a fight breaks out the interaction should be stopped immediately and started again later. Never leave the two unsupervised together as even a good play sessions can cause accidental injury. You can also use baby gates as a way for the two to "meet" each other without being able to physically be together. If fights are continually breaking out, talking to a trainer or behaviorist that can observe within the home is best.

I would keep the frenchie in the home and keep them on leashes when they first meet so you have more control over the situation. let them sniff each other through a door way, then let them see one another and take it step by step based on their reactions. If the environment is new to her, then she is unlikely to be territorial at the start until she gets comfortable. But if she is dog aggressive in general then she will have problems with the frenchie and you will have to keep them separate

Poor Merlin! It sounds like he gets very upset around other dogs, which can be a challenging problem to work on in some cases. You will need to take things very slowly when introducing him to the new puppy - this will be hard for both of them, so be patient and stay positive. For now, I would recommend keeping them separated unless you are actively working on training with them - this will help to prevent any setbacks that may occur if they get into a fight. I would try to do several short training sessions each day if you can, no more than 5-10 minutes each. You will need another person to help you, and both dogs should be on-leash. Start with them on opposite sides of the room, and give treats for calm behavior while they are in the same room together. Over the next few days, if things are going well, you can gradually start to allow them to be closer together, still giving treats for calm behavior - if either dog becomes upset and starts to bark or growl, just move them further apart again. It may take several days or even weeks before they can safely be loose together, depending on how severe Merlin's problem is. If you are not seeing any improvement within the first week of starting this plan, I would strongly recommend getting in touch with a good reward-based trainer who can work with you in person. You can search for a qualified trainer in your area here: www.ccpdt.org