I'm not aware of that pygocentrus ternetzi are protected...well actually the ternetzi that we are know of is actually pygocentrus nattereri, just a different variant of it. it's not as common as typical wild reds as they are only available from few locations. you can find larger ones available (Shark Aquarium) but not baby ones.
what you have are most likely not serrasalmus spilopleura either, they are actually serrasalmus maculatus. serrasalmus maculatus are often called gold spilo or gold piranha but they are not spilopleura; that's two different species. macs are much more common than true spilos.