I have three of these guys, will usually max out at about 4". There pretty cool and interesting but they very rarely move. Mostly just to feed. They can be hard to train to eat anything but live fish (fry) but I've got mine to take krill from forceps so it can be done.
I say go for it, 20 is a decent price. Just be careful of tankmates as they may be stung if they harass the monsterfish and eaten if their smaller. In community tanks, T.Amazonica can often starve as they wont compete or move for food, they'll only eat if food passes directly overhead.