Simply hot dark matter is not allowed due to free streaming. So do gravitational waves: a) free stream if not why not?-Surely they can since they are relativistic an weakly interacting. b) if they free stream is it allowed because they don't dominate the dark matter content?
Gravitational waves do free-stream for just the reasons you give: because they are always traveling at the speed of light, and they have an extremely low probability of scattering. I don't see anyway in which it is connected to the relative contribution of dark matter.