i have a movie clip with layers of animation isnide of it with key frames. how
can i make the animation go slower without changing the fps of the movie? can i
somehow "stretch" the timeline inside the movie clip (or layer by layer) so
that the key frames will distance themselves from one another proportionaly? Or
is there a better way to do it?

Yeah, been there. If I understand right...

I found that I should have planned better and used some better practices
(still learning). What I had to do was to create the animations within nested
movie clips. Then each animation could be slowed down by inserting frames
between the keyframes as needed. It took some thinking, but not unreasonable
effort.

If you don't already have nested movie clips then you have a bit of copy and
paste efforts to do. Just create a new movie clip on the stage, then copy your
animation layers from the main timeline to your new clip. Convert existing
layers to guides for now. When you have your animation running at the desired
timings, then you can delete your original layers. Use AS to stop the main
timeline, play your animations, and return to the main movie.