You can still try a jawfish. My concern is diamond gobies get so huge. I have seen some really large ones at my LFS
that in my opinion need a 3ft tank minimum. However, you can try a small one and see how it goes. You can get a small one for under $20. One thing for sure is you are going to have some serious sandstorms in your tank between those two fish. Haha. I should mention that I considered a diamond goby for my 3ft 40 gallon and talked to an LFS
employee. His concern was that diamond goby could damage some of the corals on the sand bed by dumping sand on them. I would start with a blue dot jawfish because it is a really cool and pretty fish...worth every penny. It offers so much more coloration than a diamond goby. Then you can build around it. It's like having a star player on a sports team and building around. Haha.