I thing that puffers would go after invertebrates, depending on how big the puffer is compared to the fiddler.
I've kept fiddlers with shrimp without any problems. There are many species of shrimp that handle brackish water well. If you keep multiple fiddlers you need to have enough room and hiding places for each when they molt. They (particularly males) will go after each other if they get the opportunity.
Monoes get 1' tall and need 300g+ tank, archers get like 14", puffers will eat him most likely, "columbian catsharks" need large tanks (18") and will prolly eat him.
Mollies are fine. So are shrimp (amanos would be cool, may even breed)...