New German Sheperd puppy and smaller dog

Question
We recently brought home a German sheperd/Chow mix puppy from our local Walmart parking lot. It was a total impulsive thing to do and was not well thought out.
We have a Chihuahua at home who is never alone with the shepherd and a small dachshund(sp?) mixed with some sort of terrier.
The small dog and the puppy (about 4 months old) get along very well but now the puppy is totally dominant over the littler one and plays very rough.
How do I teach the puppy not to hurt the little dog?
Is that even possible or do they need to be separated for the safety of my smaller animal?  

Answer
First, I'd suggest that you enlist the help of a local trainer, Rachel, because it's very difficult for me to help you over the internet, without being able to see the behavior of both dogs in person.

Having said that, I would recommend that you teach the puppy some basic manners (sit, lay down, come when called, and walk nicely on a leash) so that you have some control over him, because he's going to be a big dog when he's grown. I'd say around 60 pounds if he really is a mix of the two breeds you think he is.

Keep him on a leash (you can just let him drag it around) whenever you're at home with him, and when he starts getting too rough (ie: when your smaller dog starts behaving submissively and/or trying to get away), tell him 'Enough!' in a firm tone of voice and give a quick jerk on the leash to get his attention and make him stop. If he doesn't stop, you didn't jerk hard enough. Jerk again, slightly harder, and repeat until he stops. Then praise him, and call him to you. Praise him again when he comes to you. Then give him a chew toy or something to keep him occupied, so he doesn't go right back to the smaller dog.

I would NOT leave the dogs alone together. With the size difference already, he could easily hurt them, even just by accident.