In this case it is probably just your puppy playing. A puppy which clearly sees another dog as a leader will still attempt to jump on that dog and hump if allowed to. Puppies push limits and have no manners even by dog standards.
As to the second question. Be very careful about introducing your puppy to new dogs. One bad experience could turn the pup dog aggressive. As for the going on walks and being afraid of barking this is common and usually just something the dog will grow out of, especially if the puppy ever sees you defending him against dogs that are pushing him around or attempting an attack. Puppies take immediate notice of their owners defending them and they respect the **** out of it.