Fiddlers can't go in freshwater aquariums. They require both brackish water and land access.
My rainbow shark doesn't eat or bother shrimp and other inverts, but many will. As will mature swords and mollies. If you haven't had ghost shrimp yet, I'd do a test run with those first since they're cheap. If they get eaten quikly you might want to avoid investing in other shrimp. If they last, you could try any dwarf shrimp like RCS, blueberry, blue pearl, yellow, etc. Or dwarf crays like CPO's.
2-3 bamboo shrimp or a vampire shrimp would work in a tank that size if it were planted, but I probably wouldn't go for them since you don't have much in the way of the live plants. I can't say how well they would work in there since I've only kept them in fairly heavily planted tanks. If you did decide to go for filter feeders, you'd need to keep an eye on them to be sure they are getting enough to eat. They should rarely be moving around on the substrate, picking with their fans. If they are, they need supplemented with microfoods they can filter from the water.