Just an update, I finally got my 8/12mm inline diffuser today and it fits the zoomed 501 tubing perfectly.. no modifications or different tubing required.. which makes me happy since I was worried it may be too small, but it definitely wasn't
Now that I know it works I just have to order another one (or two).
I have all 3 sizes of the inline diffusers now, the 16/22 on my Fluval 306, the 12/16mm on my Finnex 360, and now the 8/12 on my ZooMed 501. The two bigger ones have the same size diffuser but with different connectors, the 8/12 is much smaller overall.
Thanks for following up! I'm going with an Eheim 2213 instead and I ordered the 12mm GLA diffuser. I also got carried away and got a 1/2" dia. Hydor ETH... And some Amazonia... And some plants... And some Lilly Pipes. Don't judge me - I'm stressing out waiting for graduate school admissions decisions.