Hi, I also have just two months ago adopted a 4year old male Yorkie, 'Homer'. I have two other dogs (a 5year old Labrador & a Setter) and its a challenge every day for Homer not to attack the Setter. Homer was also
marking everywhere (laying out his territory!!) So we had him neutered two weeks ago and the marking has definately STOPPED

The agression against my Setter hasnt but my vet assures me its early days & may take 3-6 weeks to see any change. So go ahead & have the little fellow neutered as it may help with his marking.
If anyone has any advice on how to stop Yorkies being aggressive with other dogs in the same household it would be greatly appreciated. Poor Homer comes off worse in the fights but he doesnt seem to learn. I'm afraid the next attack may be his last