Exodons are about the only dither that works for me.Even with a group of 8 and even more some times they still seem to pick off one every few months.Last night I lost one of mine to my flowerhorn.He seems to get one every six mths or so.Any other dither I believe would not stand a chance.
Depends on the dithers and the Devil. That is, it depends on your dither choice, and the temperament of your Red Devil...but also there are other factors such as tank decor, and the number of dithers that you keep.
For what it's worth, I have giant danios, and "three spot" gouramis in my 125g with a variety of CA cichlids, including a female RD.
My midevil lives with a convict. keeps him occupied. Over the summer my friend had to keep him while I resealed my tank, and he ate about 3 smaller giant danios in 2 weeks. To be fair it wasn't a 90g tank. I feel like any fast, schooling fish could survive if you had a nice big floating plant.