I am trying to find adjustable speaker stands for my 9" X 9", 13 lb. bookshelf speakers. The top plates are usually much smaller than the speakers. Can I mount the speakers on 6 X 6 top plates? Won't they wobble and fall over?
The 6x6 top plates are fine...unless you play soccer in the same room they will stay on...if you have any doubt you can use a little blue tac between the speaker and the top plate in the corners and you will be fine. The speakers will likely sound better that way as well.
I almost always use blutac to locate speakers on a stand.
In some rare cases it's possible to specify a different top plate for a stand - check with the manufacturers. Most people think these things look smarter when the top plate is smaller than the speaker, and I'm inclined to agree with this. That said, the top plate should not be too small by comparison to the speaker base. The top plate should always be greater in width and depth than half of the speaker base as a rule of thumb, so 6x6 under 9x9 is fine.