Please continue to work on this. I have a smallish dog (a 13", 19 lb beagle) that I take to classes. She was raised around a large GSD, a medium sized dog and is comfortable around large dogs. In fact, our trainers use her to help shy, fearful, etc dogs socialize.
But it amazes me how many big owners don't have a clue that their dogs have prey drive for small moving things, and this often extends to small dogs. There's a woman in our agility class that thinks her dog Harvey "likes" Meri. No, her dog is stalking Meri. The first time he saw her, he tried to corner her, but I body blocked him. The owner's response "oh, can they meet?" No, they can not. The instructor is aware and has tried to work with the owner, but she's clueless. (The instructor whom I know well also knows that I can handle this myself. )
Last night, we set up for a run in class, and Harvey was allowed to sprawl out right where the run started. I set Meri up way off and to the side of where the run started because she was concerned about Harvey. The owner protested "but he's being good!" Yes, but he still thinks my dog is a squirrel, and my dog intuits that.
As a large dog owner, I get it that some large dogs see little dogs as prey. As a small dog owner, I "feel" it. And it's kind of scary.
So, it's really important to keep working on this to have a well socialized dog. Your dog may not encounter a cat or squirrel in a class, but that Chi, Pom or small beagle will be running in an agility class or at the dog park. He needs to have a solid "leave it" for them too.
Physically separate your cat from your dog in the house if you can't get your dog to leave your cat alone. Take obedience classes or lessons to learn to train a rock solid leave it. And if you KNOW your dog has that sort of prey drive, keep her leashed or kenneled at all times. Dogs that chase critters can get hit by cars (or in some areas, if they trespass on other people's land and harrass their animals, they can be shot). Dogs with uncontrolled prey drives may chase larger prey and be seriously injured.
Whole Dog Journal has an article this month about getting dogs and cats to live harmoniously together. http://www.whole-dog-journal.com/issues/
If you don't subscribe already (I think every dog owner should. It's such a wonderful resource), this might be just the right time to consider it. I believe you can just buy individual articles though.