Your pacus will be fine in a 180 for a year, and you do not need to pad the sides and back to prevent them from breaking it. Even with a glass tank, the fish itself is unlikely to be able to generate enough pressure at the point of impact to crack the tank (fish are pretty soft and squishy when you get right down to it); most broken tanks are the result of the fish knocking something else harder and sharper (glass heater, rocks, etc.) into the glass creating a lot of pressure on a very small point of impact. And acrylic is even less susceptible to this problem than tempered glass.
But your fish will eventually want a lot of room in order to feel comfortable and less skittish, so the larger tank will eventually be a very good idea, more for the health of the fish than the tank.