ok, the name spilo is probably the most confusing name for piranha, it's been used with three different species of piranhas.
Serrasalmus Sanchezi aka red spilo
Serrasalmus Maculatus aka gold spilo or gold piranha (this is most like the ones you have, they are known to be raised in captivity other than rbps)
Serrasalmus Spilopleura aka red ruby spilo (this is the true spilo I'm referring to)
Click the links below to see the difference.
gold piranha
http://www.aquascapeonline.com/products/gold-piranha-2-serrasalmus-spilopluera.html
true spilo
http://www.aquascapeonline.com/products/ruby-red-spilo-4-5-very-agressive-rare.html
Here are more info on OPEFE.com
http://opefe.com/maculatus.html
http://opefe.com/spilopleura.html
I'm not saying I know everything, I could complete be wrong and what you have are indeed what I referred as true spilo (at least I have not heard or read any information of them being breed in captivity). only time will tell but I'm just staying my opinion.
and I wasn't even directing my previous common to your fish, by gold piranha I automatically link it to Serra. Maculatus (and I complete forgot about that "ternetzi" topic) and was just suggesting a rare and beautiful fish for your 50g.