I'm sorry to have to tell you, but this tank is too small for neon tetras - they need a tank at least 60 cm long. Tetras are active fish which need a fair amount of swimming room.
Depending on whether you have soft or hard water, a 19 litre (5 gallon) tank is suitable for a betta (soft water) or a dwarf puffer (hard water).
To answer your question on mixing tetras - as long there are at least six of each species, tetras can be mixed, but they need to have a group of their own kind in the mixture.