That is correct, Periodic and Bloch Boundaries are used with non-diffracting plane waves to approximate their infinite curvature.
About your second point, I will think about it. Typlically, I would use mode source with PML boundaries, Bloch/Periodic are not required. But since this this is an infinitely wide slab waveguide, I am not sure if this will yield accurately results. Typically, when we inject a mode source the fields should decay sufficiently enough at the edges of the simulation domain (as they are localized 2D modes in a waveguide). I have seen situations where results become inaccurate when there is a large magnitude of the fields at the edges (which will happen here). But I will spend some time on this and get back to you.
In the meantime, would you be able to explain what is the motive behind the simulation? For instance, are you trying to mimic experimental conditions, or validate a theoretical framework? I am asking because it might help us identify what options we have. Thank you!