What version of EX9200 SW are you using? I would think you need to be on 14.2 or higher. I assume you are saying that the command:
set aex aggregated-ether-options link-speed mixed
Either errors or does not commit, yes? I do not believe there is any [SW/commit] restriction that would not allow a mixed speed AE to be formed, but I also believe there is nothing in the hash algorthum that would take into account speed of link without above knob being set.
I really think this should at least commit and work, now work-well (and supported) could be something else.
Have you tried it?