C series gears

[TMX26]

I’ve looked into this and I can’t seem to find any information about interchangeable gears between C series gearboxes - only interchangeability of the gearboxes themselves.

Specifically for the Yaris 1.5 TS, I find the 4th gear ratio too long compared to the 3rd, and the 5th gear too short compared to fourth. The solution would be a shorter 4th gear.
As far as I know, the 4th gear ratio on the original 1.5 TS gearbox is 1.031. Fitting a shorter OEM 4th gear from another C series gearbox would be an obvious solution.
But for even closer ratios, I was wondering if one could swap the first 5 gears from the C160 gearbox bolted to 20V 4A-GEs into the gearbox of a 1.5 TS (since both are C series gearboxes). The 1st 5 ratios of that gearbox are very close, while still allowing the car to achieve its top speed (loosing only a little in fuel economy). Is this possible ?

Again, I am sorry if this was asked many times before. Thank you for your responses, anything will be appreciated.