The number of fish any aquarium will safely accommodate depends upon several factors, many of which are frequently forgotten or more likely not even recognized. Obviously the size of the fish and the number is important, but so is the species particularly when you begin adding more than a single species. The environment is a major issue, meaning the water parameters (fish react to these in varying ways), the aquascape (many fish need this or that), and of course their behaviours (active swimmers, sedate fish, fish likely to nip, territorial fish, etc). When fish with differing needs/parameters/environment are combined, there is a greater impact biologically.
Considering the tank size and your present fish, I would suggest staying with sedate fish to match the existing. More cardinals is good. Rummys are a good tetra, though I would like to see them in a longer tank--I assume the Roma 125 is 80cm (31 inches) and this is not much length; rummys are more active swimmers and will be happier with more length to swim. Ember are fine, and a group of 12 would still allow you another shoaling fish. I don't know your water parameters, but if on the soft side you have the option of hatchetfish (the Carnegiella species like the marble are quieter) and pencilfish (some would be ideal, others less). There are other quiet tetra that remain on the small side, like the Loreto. Another thing to keep in mind when considering fish is the level in the aquarium that they tend to inhabit. Cardinals are mid to lower level fish. Finding some higher level species (one reason I mentioned the hatchets, and pencils which tend to be such) will make things more interesting.
Some corys perhaps for the substrate area; no issue here with the BN. By the way, male BN are territorial so provide lots and lots of chunks of wood to break up the space. The wood will benefit the other fish being considered too. As will dried leaves.
I'll leave it for Chris to respond about the rams, except to say that you really wouldn't want two males in this small a space; a pair being bonded male/female would work, or you could look at the similar Bolivian Ram which as a solitary fish makes an ideal community tank citizen.
Byron.