I look at it this way: on the one hand, they're living creatures, and my guests. On they other, from an aesthetic standpoint, it's like painting or mosaic or sculpture or lapidiary. Picking the right number, shape, color, size, and activity level of fish to balance against the aquascape is as much an aesthetic choice as finding the right gems to put on a silver bracelet. The choice is different if the bracelet is gold or jade or wood. Do you want bold contrasts spread out a bit, then neons. Do you want a more muted, pastel effect, then flames. Are you looking for a warming effect - gold von rios (xanthic flames). Tight cluster of tiny bright red spots in constant motion? Rummies.
Frankly, neons AND flames look better together than either does separately. 10 or so neons and 15 each Flame and Gold von Rio would make a great show in a 55 over a medium grey substrate with sags in the foreground, swords in the back center, and stems in the back sides. Choosing black lava rocks for the hardscape, with long, branchy driftwoods providing structure to the plantings could really make this beautiful. Some otos (7 or 8) and kuhlis (6) round out an aquarium as artwork. (Stick a pair of german blue rams or double yellow cockatoos in there after 6 months and ooh nelly would that pop.)
/ BTW - married with kids. I just know how shape and color work, and can get fabulous when carried away.
Logic is only the beginning of wisdom