Hi there!
Do you need them to set up optical links? As long as the optical modules on the interconnected line boards emit the same wavelength, the same rate, and the same FEC attribute, they can set up optical links. It is not necessarily related to the version of the device.
An issue to be aware of. The OSC or ESC is used for networking. If the OSC is used, an OSC board needs to be installed on the OSN 1800. The DC908 does not need to be installed because the DC908 integrates the OSC and ESC. If they are all gateway NEs, you do not need to consider this issue.
Good luck!