I would also be very wary of any breeder who advertises "teacup" dogs. It's a marketing gimmick, and the dogs are often not bred with health and temperament in mind. They're bred so the breeder can make money. Quality is not top of mind for them. Therefore, you aren't really guaranteed a certain temperament, so it would be difficult to gauge how much exercise the dog really needs.
Smaller doesn't always equal less exercise. There are some 10 pound dogs out there that could probably put my dog to shame!
As to answer your question on exactly how much exercise for a herding breed, I would put it at least one hour minimum during the week if you're just doing leashed walks mixed with some training, plus something more strenuous and fast paced on the weekends to really burn some energy. An obedience or agility class would also be very helpful to keep their mind focused. It really depends on the dog, though. There are mellower Aussies who could probably be fine with a little less, and higher strung Aussies who need significantly more. A good breeder would be able to match you with an appropriate puppy, or tell you if her stock won't fit with you at all.
To put this in perspective, I have an Aussie/Collie mix who I would consider medium to high energy. When we lived in town, I would go for at least a 45 minute walk every day after work, but he was on a 100ft long line and we walked through parks where he had plenty of opportunity to canter along, sniff wildlife smells, and move in a giant circle around me. That's a bit different than a walk on a 6ft leash. We had an agility class once a week. On the weekends, we would go on longer hikes in more secluded green areas that lasted 1 hour to 1.5 hours. We had a small yard that he was allowed to lay about in when we were home and watch the world go by. I would say he was satisfied with that amount of exercise and stimulation, but if for weather or life reasons exercise dipped below that, he would get pushy and restless.