Antti, Ah, yes, language barriers, always an issue online.
Very antisocial dogs can still be socialized, it's really never too late. The difference of course is who they are allowed to socialize with and where. They should socialize with dogs their own size who can also "take care of themselves". I would not let a large unsocialized dog attempt to learn socialization with smaller dogs. They need to be with dogs (plural, ie multiple other well socialized dogs of the same size or larger) that can teach them the proper socialized play and manners. This is essentially how young dogs learn, from the older generally larger dogs. These older dogs teach them through example and by maintaining their proper behavior in play. As for where, it should always be "neutral" play space. A park or field that no dog feels the "home space" attachment to as this can change the psychology of how a dog reacts.
Many of my friends have asked me to help them with their dogs, knowing that I know what I'm doing, but also that they can trust their dog with my dog, and my dog will help the socialization of theirs by properly correcting their dog. Granted, my dog is 70lbs so he has a size advantage over most other dogs, and other than stronger dogs like Rottweilers and Pitbulls, has the strength and speed to more than take care of himself with others. That said, my friends that have those types of dogs are also excellent trainers and so he has a few friends of those breeds and they get along great (actually, my one good friend has a Rottweiler who I like to say I "cheat" on my dog with, whenever I'm able to spend time with him I can't help but want to only play with his dog...).
When you have this combination of size and proper socialization, you need to let the dogs play out on their own. They know their behavior better than humans, they know how to read dogs better than us, they are quicker to react than us, etc. Yes, there is always the chance of harm, but if you have, say, 3 well socialized dogs with one that still needs to learn, the numbers will favor the socialization behavior and they will keep the unsocialized dog from being overly aggressive in play. This is the most difficult part for most people, as they really don't understand what constitutes play and what constitutes real aggression. Most people think many of the play behaviors are aggressive and mistakenly remove their dog from the situation, this is obviously bad.
One way the owner of the unsocialized dog can help the situation is to tire the dog out before these play times. Take them for a long run, get them very tired. Their lowered energy level will make it hard for them to mount much of any true aggressive behavior as the other dogs will be both physically and mentally much faster comparatively.
Mounting is part of play. Humans see it as a sexual thing (at least in America), but in the dog world it is part of play, this becomes obvious when you see that in the dog play world mounting is done by and to both sexes in every imaginable combination. Yes, some dogs are less responsive to it and so when mounted react in a fiercer manner to remove the other dog, but in proper open field play this should not be discouraged! But again, out on walks on the street is not a place for proper socialization, take the dog to a park! Perhaps find other owners willing to help you socialize your dog in the proper way, arrange to meet at a park for at least an hour where the dogs can have open space to play and run. Make sure they understand what your goal is and why you are asking them (similar to larger than your dog, well socialized and well behaved, and under the control of their masters in off-leash situations), this will help assure them that your intent is not to hurt their dogs but to have their dogs teach yours.
Some final comments on what you've said, just an in case, and no worries if they come from the language issue...
Dogs do not form "packs" in the same sense as we think of in the wild animal world. Yes, they recognize familiar dogs and can be happy to see them, but to a socialized dog, there is no exclusivity. To a dog, all other dogs are part of their "pack". Humans are not, cats and squirrels are not, but all other dogs are. This is one of the reasons Caesar Milan is successful, he allows his well trained and socialized dogs do do the work. Have 40 "good" dogs and one "bad" dog then of course you will have a high success rate, you don't even really have to do anything at that point!
You mention the use of the backpack as a signal to the dog of sorts, so you see the power that something as simple as that (or the exact leash or collar they are wearing) can affect their behavior. The problem is when you allow some behaviors to become linked to those objects. You've trained your dog to behave differently whether it is wearing it's pack or not. While this is fine and can be extremely useful, certain behaviors should be unconditional, ie whether or not the dog is wearing this specific item it should always maintain certain behaviors. This is why, for example, the advice when using certain types of behavior correction collars, that the dog should be allowed to wear it when it is not being used, so it being worn is not associated with those behavior modifications.
Finally, you mention that you train separate from playtime. This is fine, but it should not always be the case. Many things should be trained *during* play time as well so that the dog knows those behaviors are expected no matter what the situation. Plenty of times I am with my dog at the park and people call for their dogs expecting them to go to them as when they are at home and the dog will not. My dog however, since I trained him to do so in play time, always comes when I call him, in fact, many times, he will go to the other owners when *they* say "come!" expecting their dog but instead having mine arrive at their side.