Before we can advise you, we need to know what tetras do you have? I am not familiar with Cherry tetras and a search gave me cherry barbs and neon tetras
Since these are going in a 20 gallon tank, you need to decide if they are going to be the focus of your tank.
Apparently, rather than being a schooling fish as I expected they seem to pair off.
Schooling fish will only school when there is danger. When they feel safe, they‚Äôre all over the place.
The advice I always heard is tetras are best in odd groups and more often than not, the more the better. What do you see as the final fish list for this tank?