Is my puppy learning to be aggressive?

To me it sounds normal.  Some dogs are REALLY vocal.  When mine play and wrestle you'd think there's a full scale dog war going on but it's just play.  It sounds like you're supervising and calling it off before one dog gets sick of the other.  I think it sounds like normal play and if both dogs enjoy it for a while, I would let them do it, they learn how to "read" each others' signals this way.

Perfectly normal behavior and I wouldn't try to stop it.  The older dog will stop it when she's had enough.

Dogs learn best from other dogs and it sounds in this case everything is very normal for a 10 month old puppy. They are telling each other a lot more then you can see. A young puppy will go through a very unstable phase trying to sort out who they are.  They will try their limits on both sides of the scale so growling and intense play is normal when this young. As humans we sometimes miss many of the cues that are going on or misinterpret them.

Aggression is a very tricky subject to discuss and it can be argued that true aggression is rare to see.  Some aggression is inherited and some is learned, behavior vs. temperament.  Behavior can be modified and in many cases a reactive dog can be worked with.  A truly aggressive dog will always be aggressive but can be managed. In most cases when a dog is displaying undesirable traits it is reacting rather than showing aggression. There are often triggers that set the dog off. Play can lead to aggressive tendencies so it is good to monitor play to make sure it doesn't get out of hand. At the same time, stepping in too early and removing the dog from all activities he engages in can created a fearful dog.  It is a tricky balance and best thing to do is seek an expert who can guide you to understanding your dog's play style and behavior best.

For any puppy I think group play in a controlled environment is extreemly inportanting in creaing a stable dog.  Check out local dog training classes.   A trainer with a stong knowlage in dog behavior can monitor the group and help you to create a stable puppy and set you in the rigtht direction to ceated a healthy relationship with your dog.

First thing I did when my dog was about 12 weeks old was hire a behaviorst to come to my home and evaluate my dog.  In that one visit she set me in the right direction to do foundation training and create an understanding that has helped me build a stable well ballanced puppy. Eventhough I do have a stong understandng of dog behavior and background working with dog I still find that having a coach like another trainer is a valuble tool.

 Sounds like a normal play, in which puppy is also learning the social manners of dogdom.  Nipping ankles in particular is how many dogs initiate play - along with the play bow.

Liesje

Some dogs are REALLY vocal.  When mine play and wrestle you'd think there's a full scale dog war going on but it's just play.

Our first dog was very quiet and mellow, so that's all we knew.  Now we have a dog who is very vocal, very exuberant, and still fairly young.  The first few times we watched her play in the supervised playgroup offered by the local shelter where we adopted her, I thought she was going to hurt herself or another dog.  The noises they were making while they wrestled sounded horrible!  Their fur/hackles were raised, they growled, they used their mouths on each other, etc.  But the trainers assured us it was part of the play arousal behavior, and we soon saw how she learned to read the signals given by certain dogs who just didn't want to romp so roughly with her. 

When Ruby plays with our friends' dog (a middle-aged black lab who is nearly twice her size), she wears him out.  He tells her when he's had enough, but until then, he lets her bite his legs, or even grab onto his jowls with her teeth!  Sometimes it's unsettling to see, but they always rest comfortably together on the same couch when they're done playing. 

Here are some videos of various dogs of mine, playing in various ways. A few are older/adult dog and puppy. Note the differences, all are what I would consider normal and note in some of the videos I do use vocal cues to break attention or correct my older dog. Sometimes it's quiet play and sometimes loud, depends on who's playing LOL!

http://www.youtube.com/watch?v=geF54SyXf3Y

http://www.youtube.com/watch?v=SFjECfyXNIk

http://www.youtube.com/watch?v=qJwMmqgNnBc

http://www.youtube.com/watch?v=8QO15TeimX4

http://www.youtube.com/watch?v=vbc7fnjnn-s

http://www.youtube.com/watch?v=gUKHsETjKdI