Don't know anything about killis but started to search for some that would stay small enough. Found the following genera on the
killi.net where the maximum length was given. I chose those genera all the species of which stay under 4.5 cm (ca. 1.8'') - but there surely are other genera with species small enough for our nanos and even picos!
Adamas AF
Aphyoplatys AF
Campellolebias SA
Congopanchax AF
Cynopoecilus SA
Diapteron AF
Fluviphylax AF
Foerschichthys AF
Fundulosoma AF
Yucatan flagfish
Garmanella pulchra CA
Leptolucania NA
Catarina Pupfish
Megupsilon aporus CA (Mexico)
Micropanchax AF
Pantanodon AF
Plesiolebias
Stenolebias SA
Terranatos SA
Tomeurus SA
[AF = Africa, SA = South America, CA = Central America, NA = North America]