It is not uncommon for dogs not to get along, especially same sex situations. Why do you want so bad for your Dobe to be with other dogs? Why not just keep him separate if there is a problem? IMO the biggest mistake people make is forcing dogs to get along. Some dogs just don't like other dogs! By forcing them to get along you not only risk injury to the dogs but you also risk injury to the people involved who get in the middle of it. Also, once a dog gets into one fight there is usually no turning back and dog aggression will escalale and become a problem for the life of the dog.
I also read and responded to your other post regarding your dobe's nervous behaviour. This "may" be related to the dog aggression. Sometimes (but not always) dog aggression can be a fear based reaction. Often times a confident dog won't initiate any fights becasue they won't feel the need to unless they are actually being challenged. On the other hand, a weak dog is always feeling threatened and sometimes will start things to try and "scare that threat away". I am guessing this is the situation in your case.
My advice is to keep your Dobe separated from other dogs, especially other males. By putting him in these types of situations you may actually be lessening his confidence which is the last thing you need. YOU become the dogs fun, don't worry about the other dogs. IMO dog to dog socialization is over rated and not necessary in most cases.
Good luck.