Hi
Your GSD puppy may not have learned that challenging older dogs has consequences. Being bigger and stronger is allowing your puppy to win these challenges and thus it is fun and rewarding to do so. Usually the older dog will tell the puppy off by snapping and growling at it but since the pup is bigger and stronger this is not working. The best approach is to try and teach puppy that every time the Bichon comes into the room there are other more interesting and fun things to do. For instance a ball might appear or a tasty treat. However, before you give this to your pup you must ask for a sit and a wait before giving permission to take it. If you don't your puppy will soon learn that annoying the Bichon results in a treat. It has to come to you, sit and wait etc so it won't directly associate annoying your Bichon with gaining a treat only that if it comes to you and sits and waits first before being told to take it results in the fun thing happening.
You could also try all walking out of the room with the Bichon and leaving the pup behind every time it starts annoying her. She will learn that the fun stops instead and should calm down. If she stops give her something else to do.
If you start getting cross this may result in your puppy getting frustrated and even doing it just to get even more attention.
It might be a good idea to make sure that your new puppy attends a good puppy class and take a look at some recommended reading( see APBC website www.apcb.org books).
Hope this helps
Rosie Barclay