You can't. The first movie that is loaded into the _level0
timeline defines the frame rate for that occurrence of the Flash
Player. If this is on a webpage you can probably have the page
be a new instance of the Flash Player.
Other tricks would be to search the forums here. Kglad has
posted some nice code that lets a movie fake different framerates.
Or you could redo the 6fps at 25.
Personally I try and do all my swfs at one framerate –
unless I have a specifically good reason not to. In my case 21 fps.
That way you develop a feel for that rate. I know some other folks
here do similar or they have one general rate for web and another
this kind of project or another for that.