Fiddlers are BW, and shouldn't really be kept in FW. I wouldn't keep them in a 20g. The puffer might be ok, but it's kind of small to risk...Of course you'd also need to lower the water and have something for them to climb out on.
The crayfish would probably be able to kill/eat most of your fish when it's larger. Again, smaller tanks leave little space for them to keep to themselves, and they do like to grab a fish here and there
