Is lowering my ODD fan speed dangerous after installing and SSD?
So, just to be clear, you replaced the optical drive in your iMac with an SSD and I am assuming a drive caddy.

In the process, you removed the optical drive temperature sensor:



When this sensor is absent, the fans will spin up to full speed.  This is the "fail safe."  So, you are using a software utility to control the fan and essentially override the systems ability to regulate temperature.

I would try to find a way to put that sensor on your drive to better control the fan speeds as the chips themselves would heat up under heavy use. Kapton tape should work just fine.

Damage?  I don't know because I never removed this sensor and I am unwilling to experiment.