"Has anyone tried using a slotting saw to remove
most of the material and then cleaning up the slot using the form
cutter? "

This is a very old approach in watchmaking and it is still the preferred
approach for cutting pinions. Pinion cutters for clock/watches are very
dear (can't be had from MSC) and every effort is made to spare them.

In the very early days (1700) the wheel was divided with a slitting saw and
then a special file was used to shape the teeth. Later, this evolved into
the "rounding up" tool; a self indexing tool that is used to modify an
already divided wheel. When I have a wheel that does not conform to current
cutters (tooth is shorter or longer) then I divide the wheel and use the
rounding up tool to shape the tooth. Only when the wheel is too large for
the tool will I make a fly cutter.

BTW, in watchmaking/clockmaking we make fly cutters in the lathe out of
round stock. Since we use hand gravers (ala wood turning), we turn the end
of the rod to a profile that is an exact fit between two good teeth of the
wheel we are replacing. File the profile to the center line, harden and
temper. Grind relief on the tip and polish. Takes about 30 minutes.


--
Regards,
Dewey Clark
http://www.historictimekeepers.com
Restorations, Parts for Hamilton M21s, Products for Craftsmen
Makers of Historic Timekeepers Ultrasonic Clock Cleaning Solution