You can fit the diffuser but no heater. That diffuser takes up almost the whole chamber above the pump. The spec 3is different since the pump chamber is larger, which is why they both fit. You also still get the soda water with that diffuser, even at 1 bubble per 2 seconds.
*edit* saw UP diffuser and thought it was the inline one. My comments are only about the inline, I don't know if the one linked will fit, or how well it will diffuse into the actual tank. If you put it above the pump, it may not reach the rest of the tank properly as water doesn't circulate much above it. If you put it under the spounge, the bubbles will go up to the filter media, and I believe co2 is bad for the good bacteria.