Small fish tanks are a problem when it comes to stocking. I have two 18" w x 10"d x 10.5" h, one is my quarantine tank, the other used to be my betta's tank till I noticed I'd managed to crush a corner so bought him a new tank. They both hold 24 litres of water.
Small tanks are better understocked as if things go wrong, they go badly wrong very quickly. They are also awkward to stock because they are short, and even tiny fish often need a longer tank because they are fast swimmers. And most tiny fish are shoaling fish, so you need at least six of the same species.
I can give you a list of fish suitable for a tank this size, but some are for hard water and some for soft, you need to choose from those that suit your water. You can find out if you have hard or soft water from your water supplier's website, though it might take a bit of searching. Does your kettle fur up, does your showerhead gets lots of limescale? These indicate hard water. If you don't have them your water is probably soft. You would also need to check the temps they need as this does vary from species to species.
Soft to medium water:
Fish of the
Boraras genus x 6
ember tetras x 6 - these are deep orange red when they've settled in, I have some
Microdevario kubotai x6 - I have some of these as well
Norman's lampeye, Aplocheilichthys normani x6
Sundadanio axelrodi - very soft water only
Hard water:
Danio erythromicron x6
endlers male only in this size tank
Both:
celestial pearl danios aka galaxy rasboras x6
Aphyosemion striatum a pair.
scarlet badis tricky to feed; a pair or 1 male with several females
One of the dwarf cories
habrosus or
pygmy or
hastatus x6
And of course Siamese fighting fish.
If you didn't have the platies, in a tank this size, I would go for maybe 8 of
one of the above shoaling species or perhaps 6 of a mid water swimmer plus 6 of a dwarf cory species. Or 6 of a mid water swimmer and cherry shrimp. That's all.
Or one siamese fighting fish, on his own (or maybe with a snail)
In my 24 inch long, 50 litre tank, I have 9 Sundadanios, 8 pygmy cories and an unknown number of cherry shrimps, and I consider that to be a bit overstocked.