For metastatic small cell cervix cancer, would you consider adding immunotherapy up front (per a small cell lung cancer paradigm)?

I agree that it is "not unreasonable." It is also reasonable NOT to give it. Given that these agents can have their own toxicities, which can be significant, and the potential benefit is certainly not a "home run" (and the cost is enormous) I would not add an immunotherapy agent at present.

It’s such a rare entity that systemic treatment is extrapolated from SCLC.

It's not unreasonable to consider systemic treatment like extensive stage small cell lung ca for metastatic small cell cervical ca.