We have that problem too.  Gracie, Yorkie, is one year old and Sampson, maltese, is 6 months old.  When we first got Sammie, Gracie beat him up all the time.  Now Sampson is the same size as Gracie and kicks her butt on a daily basis.  Our trainer told us that as long as there is no blood, to let them work it out.  They really go at eachother, but we watch them carefully and can see signs of play during the fighting - so it could just be very rough play.  One can hope...     
				__________________    Gracie's Mama      |