I must apopogize, I have never heard of Badis bengalensis (scarlet badis) referred to as dario dario. Even on the availability lists from the breeders/wholesalers, they are always listed as Badis bengalensis or scarlet badis.
I have kept scarlet badis, though I have not spawned them yet. They were common at the store, and always sold fast when I was helping to set up small tanks. One thing I do know for sure is that they are territorial, and can get quite nippy.
If you're determined to add more fish, what about choprae danios? or marble hatchet or black wing hatche fish (smallest of the hatchets)? I would stick to a smaller fish that is more upper dwelling considering your current mixture. Another idea would be a white halfbeak, but be specific about them being white halfbeak (DERMOGENYS PUSILLUS) as some of the others get pretty large and can be aggressive, but be aware that they are live bearing, so all males or all females would be a good idea. You can determine sex in the halfbeaks the same way you would in any other livebearing fish by looking at the shape of the anal fin.
I would suggest being very careful if adding any other mid level or bottom dwelling fishes to that tank. Are your rams a male/female pair? If so, they are pretty easy to spawn, and can claim a lot
territory. They are also known to be good parents and very protective of their fry. You might also want to consider rearranging things in that tank when adding any other fish, to avoid adding an "intruder" to a tank who's territories are already claimed by the existing animals.
Have you considered maybe adding inverts instead of fish? Things like ammano shrimp, which eat algae and are also a good scavenger, or ghost shrimp, would work well. If you want something larger, wood shrimp, or sometimes called bamboo shrimp are filter feeding and very peaceful. Red nose shrimp, not as easy to find, would also be an option.