Yes, a PCI SATA card will work for any SATA device. And you are right to plan that for a slower device like an optical drive, keeping the SSD's on mobo SATA 6.0 Gb/s ports.
ONE small thing to be aware of, though. SOME motherboards and SATA adapter cards can work together so that the added PCI card can "add" its extra BIOS to the normal BIOS on the mobo and allow the system to boot from a SATA device attached to that card in the PCI bus. Some cannot. So it is POSSIBLE that an optical unit connected to a SATA card in a PCI bus may NOT be available as a boot device. It still will work just fine for all other needs.