Pygmy Corydoras
Perhaps something from Brazil might be of interest? Three species of Corydoras — C. pygmaeus, C. hastatus and C. habrosus — are often found on sale together (sometimes even mixed in the same tank) and are true dwarves, achieving a maximum size somewhere in the 2.5-3.5cm range. C. pygmaeus seems to be the most readily available of the three, but all are equally delightful to own.
A dimly lit, blackwater setup would be truly ideal here. Leaves, twigs, and minimal (if any) planting, with plenty of tannins plus a relatively calm flow of water and a soft, sandy substrate means that setting up a dedicated biotope is not only accessible but also affordable.
Unlike some of their larger cousins, C. pygmaeus are more adept at spending time in midwater — indeed I’ve often found a detachment or two smuggling themselves in with other schooling species such as Harlequins that you would more commonly expect to see in that section of tank.
Corydoras hastatus goes a step further in the wild, mimicking species of tetra and occasionally shoaling alongside them for safety.