personally i think playfighting is perfectly fine and it doesnt lead to agression. sounds to me like your pup is just learning how hard he can bite when playing, thats why the other dog yelps and then comes back again to play, the other dog knows yours is just a pup playing. went through this with my two and they enjoy playfighting with each other. its perfectly natural, doesnt sound like you have a problem to me.

the aggression with the other two dogs sound normal to me too. in the first case the other dog had no manners and must of really upset rollo and in the second case rollo was being a bit rude and upseting the other dog. this is perfectly natural behaviour at this age. he hasnt uite got all his doggy mannors yet and it will be a while before he does, hes still learning. you sund like your doing a good job socialising him with other dogs and his classes so just carry on with what your doing and he should grow up into a well mannerd dog