I have footage shot with a webcam at 15 fps and i was wondering if i can make a 15 fps timeline in fcp or should i make a 30 fps and just work with the footage as is?
As always it depends on the version of FCP you are using and if Quicktime can play your webcam files.
That being said it is not a standard setting but you can edit the Sequence settings or the Master setting to allow this.
With FCP6 you can add the footage to a 30fps timeline and it will automatically play and up-convert. The main issue I have seen with this type of footage is the GOP issues with an IBBP encoding. This can create random issues if the information changes to much. It can also effect frame accurate editing.
I usual convert the footage before I edit to the specs of my final master output. This helps with almost all of my issues as well as preventing the need to render as much.
So to answer yes you can edit at 15fps. It all depends on what your final output will be and what you are trying to do artistically with the piece.