Hi,
How small do you want? Remember that they have tiny mouths and require smaller food. Also, no larger fish
Some suggestions:
Pencilfish - most of the Nannostomus sp remain quite small. N. marginatus is the dwarf of the bunch and quite pretty.
Corydoras - C. hastatus is a mid water schooler and also quite small.
Rainbows - threadfins are quite small.
There are many to choose from, based on origin, species type, etc.
I have owned the examples, so I went with what I know.
Google will help with others and images...........