I found a great tutorial on making Flex and Flash work together.
Here’s how it works…
You can declare a function to take place on the flex SWFLoader component’s “complete” event.
in that function, you can view / edit the contents of the Flash 9 SWF that was loaded via the content property for your SWFLoader. Ex: mySWFLoader.content["element"].
This is particularly useful for allowing an SWF to set the size of the object it is loaded into, trapping events from within the loaded SWF, etc.