Rainbowfish are my favorite dithers. The large ones such as the Boesmani or Red Rainbows shouldn't get eaten. I have had great luck with them in my tanks and IMO they are way prettier than silver dollars or giant danios
Ya i'm upgrading to a 180 this spring just don't have the room or cash right this moment and I did expect the other to oscars because I only wanted one but I adopted the two albinos. But I am getting a 180 because my red devil is going in the 100g solo. Ya I don't know if the danios will get eaton, it took them 3 days to catch the last feeder minnow that I fed them a while back. Ya between the rainbows or danios.
They'll eventually catch the danios most likely. It may take awhile, and you could always replace them as they get picked off...but if you're looking for them to act as dithers they're probably too small for that anyway. Silver dollars, or filament barbs if you can get some, would work better.