It is true that most wrasse for pest control require sand bed. Perhaps set up a container with sand in it for them and hide it behind the rock in the back, or set up a container sand bed for them. Difficult to set up a partial sand bed unless you are deliberate doing it when you set up the system. Me, I like sand bed in my reef tank so I have no problem.
Fairy and Flasher wrasses does not need a sand bed and they are very beautiful. They are often short live, life span of perhaps 5-7 years.
@evolved have an article on reef safe wrasse that you may want to look at, then expanded to do search on species that you are interested in.
Good luck