How did you cycle the tank?
He stated he done a fish in cycle. Which after two months of daily water changes and parameter checking (with liquid test kit I hope) he should be cycled (fingers crossed).
From my personal experiance, I had cardinal tetras when I first started with a fish in cycle and they loved my rock which acts as a cave with multiple holes. It has only been the past week or two that they have came out of there shell and roaming the full tank.
The questions that have been asked are also important but I would ask what is the tank scaping like? Heavy plated or hard scaped? Could explain why they huddle in a corner if it is the only corner with plants or decorations.