Yeah SATA ports and drives are backwards compatible so you can run a SATA ]I[ SDD or hard drive in a SATA ][ port at obviously SATA ][ speeds and feature set.
In fact, since the P67 has 2 native SATA ]I[ 6Gbps ports I believe you can even run one SATA 3 SSD / hard drive in one port and a slower SATA 2 drive in the other SATA ]I[ port and both will function accordingly.
The first SSD / hard drive will run at SATA ]I[ speeds while the second will drop down to SATA ][ speeds even though they are both plugged into the higher speed ports.
I don't believe SATA ports function like the old IDE / ATA primary and secondary channels where the slowest device could possibly limit the entire channel.
I believe SATA ports function independently of one another.