Choosing a focal length:
Generally faster focal lengths are used for deep sky viewing while slower are preferred for Planetary viewing. Though you can shrink the field of view you can’t make it bigger once the mirror is done.
The standard mirror is often f/8 or less despite the extra difficulty.
The amount of glass removed for a 6 inch f/6 is about 0.88 cubic inches. Where as the glass removed for a f/10 mirror is only about 0.53 cubic inches.
Those bottom numbers pretty drastic f/4 and f/12 are definitely extremes beyond the average beginner
