How to stop my lhasa apso from harassing my chihuahua?

I have a Lhasa apso to and the tend to like being the boss of the house. My dog name is spike and he loves being the boss and a spoiled brat. i recently just got a cat on sunday and my dog hates it because he is not getting the attention he is used to get.Sometimes this will happen because Lhasa apso love to be the only pet so it can have attention so you should play with the Lhasa apso then when you done put it in a cage then play with your chihuahua. When your done put your Lhasa apso which is in a cage in a room. Then put your chihuahua without a cage in the same room let your Lhasa apso know that the chihuahua is a part of the family and you can't hurt it. Constantly do this until you think the Lhasa apso learned it lesson.