If your diamond guy is getting along well, I would say no to a yellow watchman---I don' t *think*, however, a diamond woul dfight with a yasha: I could be wrong, but I have a yellow watchman and some highfins that get along fine, and that's a similar pairing. You might also look at some of the very, very tiny gobies like the trimma, etc, and clown gobies, which would be to scale with your tank. Just pick and choose and don't get them all, just enough that everybody can have a comfortable living space.