I would be highly dubious of the claim that a rubber bellow has "negligible" stiffness values for axial and lateral loadings. I'd see that for a teflon joint, but a typical rubber joint is a multiply, reinforced element and you can easily get a few hundred pounds per inch for stiffness. Obviously, it's not 1E12, but I wouldn't treat is as zero either, especially for something sensitive like a pump/compressor nozzle.