Dats may not be active enough to be dithers. If your severums are not too shy I don't see the need for dithers, but as mentioned above, relatively peaceful cichlids (e.g. festivums, angels) and characins should work well with severums.
Dither fish are used for shy fish there not truely meant for stopping aggression. You need target fish. Something they see would pose a threat so they will pretty much team up against a common enemy. Good Idea would be trying some small cheap cichlids like convicts but make sure if u get more then on that there all male or all female dont want them to breed.