New Small Chihuahua in the home

Hello: I have two cats, Jersey and SiSi. We use to have two Chihuahua's and a medium size dog and they all lived peacefully together. My Son moved out with the one Chihuahua and unfortunately the other two dogs did pass away. Now, my other two boys just found a 9 year old Chihuahua. We thought our two cats would be fine, but they are not happy at all. They mainly stay in my room where I have their food and beds. I keep their Kitty Litter out by our sliding door. The other day, SiSi peed right by my head on my bed, because she was afraid to go out to her Kitty Litter. I had no idea she was that afraid! I now have the Kitty Litter in my room. The cats barely leave my room and I don't know what to do! He's a tiny dog! He has chased both of them at separate times, but never tried to bite them, They have turned and hissed, but mainly stay in my room. Any suggestions?

Dog training classes are in order if you want peace in your house and you want your poor cats to use their litter boxes.  He needs to learn to obey SIT, STAY and LEAVE IT before he is ever allowed around the cats (crate him) and when you are first working on that, the cats need their own room, even if it's your bedroom, with door closed and dog never allowed in that room.  They need a safe place to relax.  No chasing  of cats should be allowed.  You need to be the Alpha Dog here, and he needs to respect you and mind you.  It doesn't matter that he's a tiny dog to you.  To them, he's an unwelcome predator in their home.  He has teeth and he pursues them like prey.  It is not a fun game.  Stressed cats get sick and you can end up with behavioral problems and possibly vet bills if you don't nip this in the bud.

http://www.thecatsite.com/a/introducing-cats-to-dogs