Any fish is likely to eat a dead tankmate, but it is far more likely that the carcase is just well hidden in the tank. I would try to find it and remove it from the tank to stop it decaying and casuing an
ammonia spike.
As for adding additional Rummys I would add at least 3 and probably 4 to make them up to a small school as they will be happier that way.
Even without the extra Rummys though you will be pushing the
bio-load in your tank the BN although a small fish (comparatively) is a big waste producer and is not really recommended for tanks as small as 10gals. Can you give him a bigger tank (20gal)? or could you rehome him?
Looking at your aquarium info is your tank still about 1month old? has it completed cycling and do you have readings for Ammonia,
nitrite and
nitrate in the tank you may have water issues which affected your rummy