I don't recommend green spotted puffers, scats, and monos because they are better suited for high-end brackish water or saltwater. Fiddlers are better suited in light-brackish water.
Knight goby, bumblebee goby, mollies, and other small fish are the best water-based tankmate IMO
. Also figure 8s are not recommended for the same reason stated above. Fiddlers are territorial, so you may end up with problems with the territorial knight and bumblebees. Mollies are a good candidate.