First of all, pits are not guard dogs. They are too attached to humans to really “guard” you. The only things you have going for a guard dog in a pit is the fact that many people are afraid of them and will leave you alone because of the way your dog looks. Rotts are considered a much better guard dog all around, but you really need to know what it is that you are wanting and what you will be getting into. Many dogs will defend you if they sense danger or feel that you are threatened, but pit bulls are NOT standard guard dogs. People attempt to make them guard dogs, and that is when trouble happens with aggressiveness and biting, but that can occur with ANY dog when treated in such a manner.
As for getting along with a smaller dog, each and every dog is an individual. You cannot just say, “Oh well, pits get along with all small dogs.” Nope. That isn’t the case. Just like many small dogs do not get along with larger dogs. To many large dogs, smaller dogs are prey. Dogs are “hunters” by nature, and small animals are considered prey. Pit bulls are terriers (if you have a purebred pit, which most are not), and terriers were bred to flush out, hunt, and kill game. To a terrier, smaller animals are prey and will be treated like prey. I am not saying that all terriers behave this way, but it is a typical nature of the terrier breed, and that applies to even small terriers (Cairin, Norwich, etc.). That may work with the geese chasing that you are talking about, but keep in mind that either dog that you get may kill the geese. Is that what you want?
The best thing that you can do is take the toy poodle to meet large dogs and see how he reacts. You have to keep in mind that the poodle may be frightened of larger dogs, and by bringing home a large dog into his territory, that could cause all kinds of trouble. You cannot just expect all dogs to get along. Pit bulls can be dog aggressive, and many times will not show it until it is older, as many other dogs can be dog aggressive/dog reactive too.
You need to do A LOT of research prior to bringing a larger dog in to the home of a small dog. Yes, it can and does work for many people, but a lot of those dogs grew up together, took them to training together, use the Nothing in Life is Free program, and can monitor them all the time. It isn’t that simple to go and get a large dog and bring it home and assume that it and the small dog will hit it off right away. It takes A LOT more than just that.
Best of luck!