There ARE limits as far as being able to mesh the pinion and spur. IE I need a 9.5-7.8 FDR for my LRP 4.5. (just starting out, then Ill find the appropriate FDR for the track) I tried 96 spur and 23 pinion to start out as it is a larger track. That would be an 8.35 FDR. That FDR does not work in the TC5, but it does work for a Novak 13.5 using 96 spur and 42 pinion: 4.57 FDR which is perfect for my track.
For the LRP 4.5 I ended up using a 116 spur and 26 pinion for a FDR of 8.92.
BTW I am using 64 pitch gears. There are many ways to use the same FDR, so you should find a combo that works. I do wish the TC5 motor mount had the motor mount holes milled closer to the spur, so that there was a wider mesh range.