Under Appendix F to 141, 4a, it talks about the flight training requirement. It simply says you need 25 hours if the course is for an airplane, rotorcraft, or powered-lift rating. I think certain schools just want you to get proficient with the complex prior to the checkride.
There is no additional training requirement for CFI, besides the 3 hours in the last 60 days to take any checkride. If you are doing your CFI in a 141 environment, then that is where the 25 hours of training come from.