My current plan is to have an attiny per fan, each will supply PWM signals and send back rpm data to an uno using I2C.
The uno will be communicating to a PC through usb so i can manage fan curves, temps, fan RPM.
I need the 25kHz PWM because the Intel specifications are a target frequency of 25kHz, with an acceptable operational range of 21 kHz to 28 kHz
I'm sorry, I don't know which PWM mode should be used. I have used ATtiny chips, but not the ATtiny85.
Perhaps you can get more help at : http://www.avrfreaks.net
Do you have a schematic ? do you have a way to program the chip ? do you know which timer you want to use ?