there are some dottybacks species that are very peaceful and can be mixed.
not all dottybacks are aggressive.
the 3 mentioned above however, should never be mixed together especially in a 55gal.
i have so far 4 species of dottybacks living very harmoniously together.
Pseudoplesiops rosae
Lubbokichthys multisquamatus
Pseudochromis tapeinosoma
Pseudoplesiops wassi
these 4 dottybacks, sans wassi, are extremely rare in the trade and i've only seen them less than 5 times for each respective species. but if you ever get your hands on them, they are very very gorgeous and will make for an amazing addition to the reef tank. and they are very peaceful.