Replace superdrive with SSD SATA I or SATA II?

Ah! The confusion of SATA!

So lets see if we can clarify things a bit here as there are differences between what the system can offer, and what the device is able to perform here.

The systems SATA port is based on what the chip set it has. In the case of the Mid '10 iMac model has a SATA II (3.0 Gb/s) interface on both SATA ports. The optical drive its self is a SATA I (1.5 Gb/s) device.

You can see this within the About This Mac >> System Report option. Scroll down to SATA/SATA Express then Click on the chipset banner. Here you should see two entries the first Link Speed which is what the system can offer and the second the Negotiated Link Speed which is what the device is able to run at.

Just to be clear here, you can always use a device which is slower than the systems offering. You can't use a device that is FIXED at a higher speed than what the system can support. Yes! There are devices which have auto sense technology that can slow them selves down to match the systems SATA ports speed. But not all auto sense devices will work across all of the different SATA I/O speeds many have dropped support for SATA I (1.5 Gb/s).

Now what I've stated here above is all true with all dual and three SATA ported iMac's! In the case of MacBook Pro's there is a wrinkle!

The chip set Apple used in some of the MacBook Pro systems had a problem with the systems clock rate so it's unable to support SATA III (6.0 Gb/s) devices off of the optical drives SATA connection even though the SATA port speed is listed as SATA III (6.0 Gb/s). Here we need to use a FIXED SATA II (3.0 Gb/s) device. Most auto sense devices also won't work either as the ports signal timing confuses them.

I suspect what you've been reading was this chatter which confused you.