I have some Tanfo small frame witness FCG spare parts and a complete .40 upper, do Tanfo small frame mags and other parts fit the P9?

yes

Thx, There's one in a LGS and before going to see it I wanted to at minimum be able to get mags for it as it only comes with one.

You can fit CZ mags in them too, of course not all of them.

The P9s were Tanfoglio parts, purchased by Springfield, polished and fitted under the direction of Les Baer, and were the top of the line CZ clones.  There have been some minor changes to the Tanfoglio, while the P9 was frozen in time, but for the most part, they should fit.
I have a Springfield P9C in 40 that I've been on the fence about selling for quite some time.  I never found the 40 to be fun to shoot in polymers, but my polished steel P9C makes the 40 fun to shoot and is very accurate.  I'm just deep in 9mm, I don't know what to do.  Beautiful gun.  I also have the older Wonder Witness in 40.