Will the arowana hurt itself on the hardscape (rocks and wood)?

Possible, but unlikely, if you scape wisely. Arowanas are large, active and powerful fishes. Aggressive ones may try to chase and eat their tank mates. When frightened, the arowana, like most fishes, may dart away suddenly and in some cases, blindly. Scape so that (1) the hardscape is unlikely to be in the way, and (2) that if they do knock into the hardscape, injury is minimal.

Rocks can be used in the lower part of the scape. Use rocks with smoother surface instead of the very rough ones or ones with sharp knife like edges. Use roundish rocks rather than pointy rocks. For wood, instead of placing them to stick up like branches, place them so that the sharp pointy bits point are buried in the substrate, like exposed roots of trees. Try to keep most of the hard scape below half the tank. Use soft plants with tall long leaves to fill up the upper spaces at the sides or back to balance the scape.