The "leg holding" is actually called side sucking--it's believed to be a sort of obsessive compulsive behavior in dogs. No matter what you call it we see a fair amount in Dobes--sometimes it's what your puppy is doing, sometimes it manesfests as blanket sucking or toy (usually fuzzy type toys) sucking. It's not uncommon--and not disasterous except that some dogs get so into side sucking that they can actually make sores in the area they suck, change hair color (and of course leave themselves slimey--which also seems to be just the spot you reach over to pat them on).
As far as the dog aggression goes. I'd be discouraging that in a big way--at not quite six months he shouldn't be all that dog aggressive. But they can learn behaviors that will come back to haunt you as he grows and matures.
Do you find this happens when he is on leash of off leash? Some dogs, even as puppies will feel indimidated enough when on leash that if another dog approaches and they can't flee to try to warn the other dog off by growling and barking. This you can discourage--don't let other dogs get in the puppies face when he's on leash--tell him "no" firmly but don't baby him about it. (That is to say don't keep telling that's all right, OK in a sing song voice--that's the sort of attention that will encourage him in the behavior )
If he is running up to other dogs and acting agressive toward them when he's off leash try putting him back on leash and making it clear to him that growling and barking and charging other dogs isn't acceptable behavior.
If you can be a bit more specific about the circumstances that set of the aggressive behavior I'm sure that several people will have suggestion about what you can do to change it.