The pump blocks in Simscape Fluids models the flow rate at different pressure difference and speed, but themselves are "volume-less". Depending on how the pressure difference is established around the pump, the bulk modulus may or may not matter. For example, if you have reservoir blocks with different pressures as inlet and outlet, the pressure is fixed and bulk modulus won't affect any calculation. If you pump fluids into a fixed volume, with no outlet, bulk modulus will change how fast the pressure builds up and you'll likely see some difference until the pressure exceed the upper limit (and simulation errors out). If you can attach the model where you made your observations, I may have more feedback.
You can add pipe blocks between pumps and valves, if you like and if you think it's important for the engineering question you'd like to answer. Model fidelity should always be linked to the engineering goal at hand.
