Will the tank mates get stressed out by fear of the arowana?

It depends. I’ve seen a video of a tank with a rock and plant feature in one half of the tank, and a tree like driftwood with attached plants in the other half, on an otherwise flat plain of carpet plants. There was a stressed looking school of Neon or Cardinal tetras swimming to and fro between the rocks and driftwood as the arowana glided from end to end of the tank. On the other hand, in the same video, there were Yamatos and Otocinclus happily grazing among the plants and clinging on the hardscape and glass.

In my own tanks, the small tank mates are constantly alert and cautious, but do not look constantly stressed to me. In my opinion, if you provide ample havens with plants and hardscape where the small fishes can feel safe, they are less likely to be stressed by fear. They know they have places that they can rest safely in and will instinctively balance the risk between safety and feeding.