I tried my Siemens GFI QPF 40 amp breaker in my Cutler Hammer elecrical
panel and it fit with no problems. Does this mean it's compatible. I
noticed that when the electrician replaced my panel, he substituted a
15amp siemens into the panel and it worked.

Is it better to run a pony panel off my main panel for my hot tub. If
the answer above is that the GFI breaker will work in my main panel, do
I really need the pony panel.

Thanks