That will be fine.
To the couple here who does not seem to know. There are many black water areas that are full of plants. The areas where discus and angels and loads of other fish live, do not have many, if any plants. There areas where lots of tetra and dwarf cichlids can be found that do in fact have plants. Granted, they are spread out over large areas, and you may find tetras and dwarfs in areas with no plants at all. But, they do come from areas where there are lots of plants. This in no way means you need to have plants in order to have your black water bio type.
In a 20 long, if you got 6 marble hatchets, and 6 to 8 rummynose, 6 panda cory, and a single ram, you should be fine.