I would probably suggest either neons or cardinals too, though not for any problem reason; I just think they look best separately. A group of 12-15 cardinals or neons would be nice.
Serpae Tetra (Hyphessobrycon eques) is something I would not suggest. A group of no less than 12 would fit your space, but this limits other options because Serpae are notorious for fin nipping slower fish. [BTW, the fin degeneration you see with this fish is not fin rot, but fin nipping; within the group they almost always do this.] A better choice which would still give you the nice reddish/mauve colour would be Rosy Tetra (Hyphessobrycon rosaceus) or Roberti Tetra (H. bentosi).
If you still want a "larger" fish, there are some options that fit here. In the gourami, consider the beautiful Pearl; a trio of one male and two females, or five with two males and three females. This is a larger gourami but one of the most peaceful. Males are territorial, so one or two in your planted 50g would be it, plus the females, and 3-4 females is better to ensure they are not pestered continually by the males. Another larger fish is the Bolivian Ram, which does well either singly (one fish) or in a bonded pair. This fish keeps more to the bottom though, so it would not be so obvious, unlike the pearls that are upper fish. If you get the pearls, the nipping issue becomes more crucial, but with enough in the groups the tetra so far named (other than the Serpae obviously) are OK.
Byron.