A sand sifting cucumber, IMO, is a very nice addition depending on substrate (not good for more coarse types). A sand sifting star is not so good, for a similar reason a goby is not...they are eating a lot of the good creatures in the sand bed. Several types of snails, such as nassarius or conchs are good.
The basic question, though, is are you trying to resolve some sort of issue? If this is the case, then a fish is very unlikely to resolve it. While inverts are pretty much a good part of the clean up crew at any time, it is important to understand the limitations. So are you just augmenting, or are you trying to solve something?