Any tips on one more small fish?

[Congaken]

I have a 10 gal. nano reef, up since June...presently have a spot tail blenny and a yellow tail damsel...inverts, etc...Is it too late to add one more small fish that can handle itself with the other guys...putting in the blenny was no problem...as you can see in my latest post, my yellow watchman disappeared and I think that the blenny and damsel pushed him around...I know that I am about at my limit fishwise...but one tough-but-not-too-tough little fish would complete the picture if its possible...any suggestions?...Thanks...