What fish, if any would be left in the 50 litre?
If the answer is none, move the angels and torpedo together with the filter in the 50 litre all into the 200 litre, then keep an eye of the levels for a few days. If the ammonia and/or nitrite start to rise, lower them with water changes.
If there will be some fish left in the 50 litre, move some media in proportion to the two lots of fish. You'd need to go by the body mass; the bigger the fish the more body mass. Again, monitor ammonia and nitrite for a few days.
And in this scenario it should be easier to get the nitrate level down, and stay down, with fewer fish.