Hi!
I personally don't recommend keeping Siamese fighting fish with small schooling fish (like tetras). Regardless of the size of the tank, they are easy targets for aggression and bullying because of that small size.
With that being said, what is the size of your tank? That will be a big factor in determining tankmates, with larger tanks allowing for more variety.
As an example,, I've got a Siamese fighting fish in a 473 liter (183 cm long) with 6 pictus catfish and 6 clown loaches (listed in my signature). You can read about it here:
https://www.monsterfishkeepers.com/forums/threads/rescue-siamese-fighting-fish-thread.743397/
The pictus catfish in particular are not typical tankmates of fighting fish, since they have the potential to either eat the fighting fish or tear apart their fins (as rightfully pointed out by Fishman Dave in the link). Clown loaches are also not typical tank mates of fighting fish, since they have sometimes been reported to pick on them when kept together (possibly because fighting fish are slow swimmers).
However, I have had no compatibility issues with any of these whatsoever just yet. I believe the size of the tank is key to this, since Lelouch (the fighting fish) has sufficient space at all times to get away from the pictus and clown loaches should he wish to.
So as a tl:dr, I would say that you should stay away from smaller fish, but the larger the tank the larger and more unconventional the tankmates can be becuase of the more space.
Good luck.