A springeri damsel will help with flatworms, and they are one of the few fairly peaceful damsels.
I'd also recommend a smaller blenny. I have a tail-spot and a bicolor in my 80g lagoon, and they have enough room in there to avoid each other. They are super personable, and have amusing behavior, watching you through the glass. If you want to be safe with all corals, go with the tail-spot (bicolors are hit and miss with nipping at some LPS and SPS).
Do you have a sandbed? A smaller Halichoeres wrasse would also help with pest control if you have a sandbed for it to sleep in. Halichoeres chrysus or Halichoeres biocellatus are two good options.
Fairy wrasse are a gorgeous showy option as well. They don't do much for utility, but they are super colorful.
If you're not concerned about possible coral nipping, a dwarf angelfish like a coral beauty, flame, or flameback are super active, always swimming through the rockwork and looking for snacks.