How about having both. What I mean is to have one large centerpiece fish in companion with a group of smaller fish. The center piece fish is the leading star and the group of smaller fish are supporting actors/actresses. It fits both a single fish tank design becasue from a distance one notices only the big center piece fish, and only in close up can one appreciate the smaller tankmates.
To have this mixed design, the centerpiece fish must be large and peaceful and not pisivorous that will gobble up smaller tankmates. So Flowerhorn, Red Devel, Trimac, Peacock bass, Pike cichlid or Malawian trout cichlid is out, but Green Terror, Chocolate cichlid, Severim and all Vieja species are candidate for the centerpiece. I am not sure whether Oscar fit for the centerpiece because even though it is known to be a giant whimp, I have no experience if it is safe for small fish.
I am currently setting up a 125 and 75 gal tank with a single GT and a single synspillum as the centerpiece, and a bunch of smaller Malawyian and Tanganyikan as smaller tankmates. The size difference is huge. The GT and synspillum are 10 to 14 inch fish, while the Juli and Lamprologus tankmates are 1 to 3 inch fish. They are getting along well and don't bother one another, and the Juli are happily raising babies in the rocks. The center of attention is always the GT and Synspillum because size matter, but it is fun sit close to watch the interaction of the small tankmates sneeking in and out of the rock and compete food with their giant tank mate.