[legomaniac89]

With my stocking plan of 12" square of tank area per 1" of fish you'd be slightly over. Let's see...

24"x12" = 288" square

18x1.5" (max size for those tetras) + 2.5" (max size of betta) = 29.5"

29.5"x12"=354" square

So you'd be about 5.5" over the "limit".

This isn't a "perfect balance for the tank", every tank is different, but if you keep up on water changes you should be fine.

Tetras, especially cardinals and neons, take up very little bio-mass in the tank so really you should be alright.
The tetras might be happier in, say, a 20L rather than a 20H. That would give them more swimming room but it's your tank so......