I agree w/ the previous comments. If you want to continue to add fish a larger tank should be your next investment rather than more fish. I started with a 29g and an H. crispa (Sebae) anemone with a clarkii clownfish and other fish. The bioload became an issue and keeping the water clean enough for everyone especially the corals and inverts. I had to utilize nitrification filters which aren't good for corals and inverts.
To comment on the goby/pistol combination w/ your current setup/livestock I just have a couple things to offer. I have kept all the fish you have including a Wheeler's goby/Steinitz goby and tiger pistol shrimp. In a small tank having more than one goby type fish may cause one to pick on the other. Shrimp gobies may annoy the mandarin if it comes too close to its territory. Just some advice on pistol shrimp, they can be reckless with their behavior sometimes. I've had one cut open a sand-sifting starfish and injure other livestock that live around its hole. They can't see very good and will use its pinchers when scared or threatened, but most often they are harmless. In a small tank they will make a mess of the sand, sometimes they can dig very large holes or even multiple ones. They never really stop burrowing, they're always busy working moving sand around. They use little thought in where they dig too, so they can dig under rocks and in places that can cause problems. I'm trying to say if you want to have control over the layout/arrangement/placement of your tank, you may have to give up a little to the shrimp.
