I don't go to dog parks b/c of this issue. I'm tired of having my dogs attacked and it's just not worth the negative experience for me. I do think they are great if you have a well socialized/trained dog that is already exercised. I don't mean to criticize this is just my opinion after a lot of unpleasant experiences. DP's just aren't for everyone. I do understand your need to exercise your dog. Dog parks are not a substitute for structured exercise and you shouldn't just take and release and amped up unexercised dog at a park because the result will always be less than ideal. Many bad situations start b/c owners take their dogs to the park solely for and instead of exercise and just unclip the leash and don't really supervise what the dogs are doing.
Doing interactive things with your dog at the park are good ways to distract and prevent unwanted behavior. Working on your recall to make sure he'll come when you want is a good way to avoid situations too. Your high energy hunting/coach dog mix would benefit from 2 hours MINIMUM vigourous exercise per day. Under exercistin any of those breeds will result in a frustrated over the top mentality. I have Goldens which are also high energy hunting dogs. I can tell when they need more exercise because they get really annoying and frustrated.
I suggest exercising your dog very well before you go to the park. I would also unleash the dog right away (muzzled if you are worried) so that he won't be at a disadvantage on the leash. Leashed dogs at dog parks are against the rules in many areas because they are the objects of attack since they can't interact appropriately and may start or invite unwanted interaction. If your dog is biting and fighting he does not belong at the dog park now. There are loads of places to exercise your dog that do not expose them to situations where they will be aggressive.
If your dog is aggressive with any dog at the dog park then you shouldn't go unless you train and correct the issue. I would be angry and ask you to leave too if your dog attacked mine. Not to criticize, but having my dogs attacked unprovoked by repeat offenders at parks is very maddening. Sure you may be able to train around this and I certainly encourage you do do so/ There are just many dogs there that can be injured by your dog, esp small ones, not to mention many people that insist on taking young children. The more he does aggress or experience aggression the more habituated the behavior will become.
The alternative is to address the behavior by training and would be fastest and safest addressed by a good trainer or certified behaviorist. The reason I'm so conservative on this is because all it takes is one bite to the wrong dog or person and you potentially lose your dog if they are declared dangerous.
As for meeting other dogs I would get them all really tired with separate individual exercise. Then take a long walk side by side with enough distance that both are non-reactive. This gives them a chance to check each other out without a lot of intense contact. Then maybe leashed you could let them get closer being sure not to let leashes tangle. Tangles can be a provocative fight starter. Have one person drop their leash should this happen. Maybe after the walk both can lie down near one another and check each other out in some neutral territory. If there is any sign of reactive behavior increase the distance between them and reward for a return to calm behavior. This might take once or a few times.
After you feel comfortable that they are calm but curious about one another you might let them off leash maybe muzzled. Watch for any warning, reactive, defensive, aggressive, or uncertain behavior. I might just drop leashed and let them drag. That I can quickly step on or grab it and calmy walk away from any sitiuation I don't want to escalate. Stiffening, head over shoulders, tails up, ears up and forward, commisures of mouth puckered instead of relaxed and open, any philoerection, vocalizing, lunging, picking up one paw, yawning, scratching, shaking, hard stares etc. If they do any of this you should evaluate why. Give them something to do initially like come, sit, heel, etc for treats and they'll get involved with you and pay little attentio to the other dog. Getting used to this feeling is what they need. Sort of a mild interest that is easily distracted from the other dog. I would also do the intro's 2 at a time, not the entire group and work up.
We have a reactive dog in our class and my puppy is very sweet and exuberant. She doesn't have an aggressive bone in her body but her enthusiasm makes one reactive shy dog nervous and she's fear aggressive. You don't say why your dog agresses either fear, anxiety, resource guarding, etc. My dog will try to play and this dog will raise her hair, lift her tail, stiffen, turn away, growl, hard stare and a few things to say that you make me nervous and you come on too strong. So she's warning my dog to stay back for a bit and my dog in her excitement is ignoring these signs. She'll rush in when she should give some space and time and the other dog growls.
It's our human job to slow things down a little until this shy dog feels better. Rewarding them for looking calmly at one another and walking past one another are good things for exposure. Keeping them below threshold and going slow works best. So while they may eventually have fun playing we can be more sure of that if we prevent a bad experience while one dog is worried. If we go slow and associate each dog with good things the emotional experience is made much different and positive.
Have you thought about taking your dog to a training class with a positive trainer to help socialize him in a controlled safe environment that classes offer? When you understand why your dog is aggressive you'll better be able to address the problem. Most responsible breeders require this for their puppies because it ensures a happier well socialized dog. This goes for mixed breeds also. Dog class is a lot of fun. I've been taking classes for 5 years now and both dogs are much more self confident and happy to see other dogs/people. Good luck, I think with some homework and training you can improve what is happening. I would go to a behaviorist or + trainer to get some ideas and tools to have under your belt to succeed. It usually makes things much easier. :dogwink: