I've introduced several pairs together by adding new individuals to established individuals, They allways pair and always spawn. Awsome reef fish!!!!
I use an isolation container for at least a week to acclimate the individuals and then let the new one go. I always add a smaller individual to the established fish. They will change sex, if needed, this can take some time and cuase aggresion but even then I've seen succes in pairing.
I think you can do a pair in a 28, just be slow with t eintroduction, isolate at least a week. I'm hoping to add a second pair to my 150 soon.