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Starting with a diamond watchman would be an excellent choice.

As to the dwarf angels, a 55 is a fine size for them (you can only have one, though). And you have a 50/50 shot of them going after corals. The only one I have ever had a problem with was my lemonpeel, who would only eat SPS.
Just my 2 cents, maroon clowns are downright aggressive!!! There could be people here that haven't had any probs, but nine times outta ten that fish will be brought back to the LFS or sold on craigslist sooner than later. Understand clowns are in the damsel family, which means they can all be aggressive. Basslets are another one. Pseudochromis family...mean,mean, and mean!!! if you want clowns, stick with percula or occelaris. if you want a similar looking basslet, get a royal gramma. Managing an LFS for a year, besides preadator fish, the maroon and tomato clowns were the most often returned fish to the store!!! Hell 75% of them we never sold to the cust.! Hope that helps!:) BTW the gold headed sleeper goby is by far my favorite sand sifter!!! Peaceful, hardy, and beautiful! And I believe you can easily have a dwarf angel in a 55 gal w/no issues, but the more rock structure you have , the better!!