I absolutely love my fiddler crabs, greatest little clean up crew around and, like most inverts their size, the bio-load isn't bad. Still when considering bio-load they should be recognized as part of the stock, same for the ghost shrimp which get quite large. I've been keeping fiddlers for years and I still never tire of watching them. I originally set up my first brackish tank just for them and they are totally worth it IMO!
They are not fully aquatic crabs, as you know, and must have an adequate land area to dry out their shell on a regular basis or they will not survive for long.
Please realize that these little guys are wild caught from mangrove swamps and are absolutely brackish. They will not live long in freshwater. Also adding aquarium salt to a freshwater tank does not make it brackish, and is almost never a good idea for your freshwater aquatics except in extreme cases to treat illness, and should never be considered for long term use. Your brackish creatures should be in a brackish tank and your freshwater creatures should not have any added salt.
I understand you think they are doing fine but I can assure you they are merely surviving for now - and there is a big difference between surviving and thriving. In your conditions they most likely won't last more than a few months where as if you keep them properly you could potentially keep them for years.