Clown tangs can be huge a holes so I'd definitely make it the last fish. You could do a shoal of anthias with the fish you are adding as well as some smaller rock dwelling fish and no tanks complete without a wrasse or 2
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20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"
Skip the clown tang. They are surge water swimmers and can get quite aggressive. I'm almost sold on them needing to be in a species only tank.
That said, you can easily replace it with a tank that size. I'm kinda surprised not to see any of the sailfin variety of tangs on your list as they are some of the more vibrant in my opinion.