If you obtain wild caught green neons Paracheirodon simulans they will be decidedly unhappy & will disappear as you approach the tank (or even enter the same room)
I absolutely would not keep these fish in anything smaller than a 60P (Seriously Fish suggests a 45 x 30cm minimum) and would consider that a major compromise
Keep a large shoal of these fish in a large tank with loads of plants or a black water tank if you want to observe their engaging behaviour (they are much more active swimmers than P innesi)
But then you might also buy green neons Paracheirodon innesi ‘green’ which are likely a hybrid (tank bred) as they seem somewhat inbetween in terms of behaviour and size and appearance
Though again I’d recommend at least a 60cm tank
Having a fish school in “typical behaviour” is often dependent upon large numbers and a suitable large water area (or adding a predator to the mix- not something to attempt in smaller Aquariums)
Harlequin rasboras display quite interesting (sophisticated) behaviours in larger groups (8 is a minimum but 20-30 is much more interesting) but again unless the swimming room is sufficient, only limited behaviours will be displayed
In a smaller tank, choose fish that you like as individuals- color/displaying (individual) behaviour
eg, Black phantoms are lovely engaging fish even when kept in small tanks in small numbers (I’d keep 7 as a minimum, 3-4 males, 3-4 females) as the males will frequently show off for the girls, mock battle the boys ...
Behaviour Test your Betta, if he goes absolutely mad for a mirror or pen, tankmates may take some trial and error (& might end in none suited to your tank set up and your Betta), a less active Betta is usually more accepting of tankmates (but then you need to take care that he isn’t bullied)
Of course all of this is much easier with multiple tanks - at the least, keep a suitable container at hand for your Betta ... if he seems much happier during his container interludes, that’s an answer
Most Bettas and Corydoras seem happy enough together (for a smaller tank, choose smaller Cory species)
Very small fish such as lampeyes and clown killi’s can easily become tasty Betta treats, so watch for declining numbers
Ember tetras with their deeper body are less likely snacks