The problem with pairs is that not all fish form bonds with one another so it can be tricky to maintain them together long term. For example, for me there is nothing better than seeing a couple of juvi halichoeres wrasse swimming in tandem but once they mature in a couple of years this can turn into war. Many fish get around this by living in much larger groups where aggression is diluted more but there are many species that once paired make excellent longterm partners. Anthias are a good example of group forming fish and it can be very difficult to keep them as a pair only, but dilute the aggression with numbers and they become easier.
For pairs good examples are gobies like yellow watchman, wheelers, orangespot etc which can also be paired with a pistol shrimp or pair of pistol shrimps if your lucky.
Other gobies like cleaner gobies are also great together, as can be yellow coral gobies but pairs can damage sps.
Hawkfish are great paired and share a life long bond. Scarlet hawk and long nosed hawks are the best for reefs but do limit your cuc and small fish choice a little.
Yellow head jaw fish are amazing in pairs but you need a deep substrate and a good portion of open sandbed.
Blue stripe pipefish are also great and pretty easy for a pipefish, plus easy to sex.
Other good species for a more robust tank are chrysiptera damsels like the yellow tail, azure, talboti, or Pomacentrus alleni. The trick is to buy them young when sex is determined so you get a natural pair form. You could also look at the more peaceful dottybacks like springers, fridmani or sankeyi.
