That snake in the picture could surely eat a weaned rat. Possibly even a small rat that weighs around 70-80 grams. 2 mice per feeding should keep him pretty full, but you will eventually want to upgrade to rats.
Please forget everything the pet store employee told you and read this in full: http://ball-pythons.net/modules/Sect...warticle&id=59
Also, please remove the heat rock as soon as possible. Those are well known for burning reptiles and are never safe to use.
If you would like I could give you a detailed post about what you can do to fix your setup without spending too much cash
Also, ball pythons aren't really aggressive animals. They almost always have a reason for being bitey. A reason why they might strike is if they don't feel safe. This usually happens when they don't have any hides or have hides that don't make them feel secure. Half log hides are not suitable for this species because they are so open. Rock cave hides or anything that has a small entrance is preferable.
Another reason why a snake might strike or bite is if it hasn't been fed enough (which is probably the case here). Ball pythons are often underfed. You should be feeding approximately 10-15% of the snakes body weight every week.
PS:!!