Consider some shellie species from Lake Tanganyika – Neolamprologus ocellatus, Neolamprologus brevis, Neolamprologus multifasciatus, or Neolamprologus similis.None of these species will greatly exceed 3″ in length and they’re easily sexed – adult males are larger. Except for the ocellatus, they’re not the most striking in color but they have many other redeeming qualities.
What shellies lack in size they make up for in personality. Females guarding eggs/fry are fearless. I once had a female ocellatus no larger than a big paperclip beat my hand to death when I got just a bit too close to her shell while cleaning the glass. In fact, it initially rattled me so much I dropped the scrubber.
In any case, any of these species would make a great addition to a small tank. Shellies aren’t much for the water column, so go for a shallower tank with a larger footprint. If you don’t get one custom made, you’ll have to go rimless to get one that isn’t standard dimensions (I’m talking glass here, not acrylic). But that’s not a bad thing. Rimless glass tanks are gorgeous. A good example is Mr. Aqua’s 12 gallon bookshelf tank, called Serene. It’s 35.4″ x 8.3″ x 9.4″, which provides more floorspace where the fish will spend their time. In fact, Mr. Aqua makes several non-standard volume tanks and they’re all excellent. They’re made with precision beveled-cut glass and assembled using special silicone from Germany. I highly recommend them.