Introductions of a new cat/kitten need to be done slowly. Putting a new one in the mix is asking for trouble. Especially if you have a dog who chases cats.
Poor sweetie.
Please set up the spare room you mentioned with a litter box, toys, food, water, and a little bed for the new one. Put him in the room and give him a chance to settle. Then, and only once he is completely comfy with his surroundings, and only then start doing intros slowly. It can be a long, drawn out process but if you want a quiet, peaceful home with no stress it must be done.
There are some great sites out there with lots of info on introducing cats. This is one of them........
http://www.squidoo.com/multi-cat-management
This is another one.......
http://www.wvcats.com/integrating_kittens_with_cats.htm
And welcome!