New Cat Hisses and Chases Resident Cat?

I have a 1.5 year old neutered male cat and an 8-month old spayed female dog. The male cat absolutely loves other cats. Whenever he is introduced to cats, he is very friendly and follows them around wanting to play.

I decided to get a second cat so he wouldn't be so lonely. Yesterday I went to a shelter and got a spayed female 1-year old cat. The people at the shelter said she is good with both dogs and cats. When I brought her home though, she would hiss and growl at both of my resident pets.

I quickly isolated her to my bedroom and kept her separated all of last night and all day today while I was at work. Tonight she seems comfortable with my dog and hasn't hissed or growled at her. But she is still acting very aggressive towards my male resident cat. He is not aggressive back, but keeps slowly approaching her. But she hisses and will even chase him away from her. I don't know what to do because otherwise she is really sweet. Any advice? :(

By Courtney