Aggression in a puppy

Question
3 Days ago we got a new Doxie pup (F) to train as a service dog for our handicapped daughter.  We already have a mixed breed pup (M) (lab/greyhound????).  They play wonderfully together but if Roxy (doxie) has a chew or a toy and Jameson is within 1 foot of her she tries to attack him.  With her in training as a service dog, we need to nip this in the bud before it gets out of hand.  She is very territorial with her toys-- biting, growling, jumping up and running at him.  She is 10 weeks and Jameson is just now 1 year.  She is normally very docile and Jameson is a spaz.  She is not aggressive during anything else and is wonderful with everyone but seems to have that "Big Dog Mentality"  She is this way with other large dogs also.  I worry that she will not be able to be social with large dogs if we don't fix this now.  

Just now she walked up to Jameson who was chewing on a bone and she took it from him.  He allowed this to happen and is now laying next to her watching her chew the bone with a deep longing.  

Any suggestions on how to keep her from being this way?  We have tried showing her that she will not get her way when she acts that way by taking the chewie away from her when she attacks Jameson and giving the chewie to him.

Answer
Hi Laci;
Had the same problem with a little Lhasa puppy we rescued.
He was 8 weeks old when he tried to kil out year old australian Shelherd.
Not as serious as it sounds. I hear an awful growling, and went into the room they were in. Rowdy, The aussie was sitting there with that puppy stuck on his cheek, hanging in the air, shaking like  "KILL,KILL!!!"
Rowdy just looked at me like "Do you believe this?"
I plucked him off, and scolded him, and Rowdy took him away from me, wrapped his body around him like he was protecting the innocent little thing from an Ogre.
He took over that puppy like he was it's mother, as soon as it came in the door.
The are 9 and 8 now, and Max still runs at Rowdy like he wants to fight, every once in a whiole, but he knows better than to really do anything.
I pick them up, chew their little butts out royally, and tell them they are bad and have to go to time out. Then they get put in the small bathroom in the hall for a half hour.
They HATE time out, and they have to stay in there and hear all the fun they are missing out on.
The chewing out and the time-out, gets the point across that it is punishment, and it only takes a few times to break them of that "Big Dog" syndrome.
When Max starts flying at Rowdy and grwling, I say,"MAX!! do you want to go to time out????"
And he stops and acts like a gentleman. I only use time-out for very serious infractions. I just use scolding for smaller sins.
Max learned to share his toys, and everything else.
Time out has worked on every dog I have had to use it on.
Seems the smaller ones are the ones who try hardest to take over.LOL
Charlotte