If i were you, i would just get a sanchezi because itll be happy in a 50 gallon for the rest of its life. An irritan should be fine too, they are a little bit more aggressive though. Rhoms are just way too big and keeping something that can take your hand off in less than a second when you do maintenance doesnt seem all that great.
I like it that you dissuade people from getting a piranha. I'm not being sarcastic but serious.
Piranha's are next to goldfish as the most mistreated fish in the hobby so I do my best when replying in most threads to make them not want one in my first replies in general.
The irritan and sanchezi, like you said, are both great choices for this size tank but neither is more aggressive than the other. it comes down to individual fish.
The S. rhombeus when larger is able to take your hand off at the wrist based upon their large sharp teeth and extremely great biting power but once you've had piranhas you'll learn when and when not to test them. You'll be able to tell by the way they look at you, in all seriousness, if they don't want to be messed with. Also, by the way they lean and position their bodies is another great way to tell if you should be sticking your hand in there.
Your point is correct, nonetheless, and I won't even try arguing it that keeping large Serasalmus rhombeus can be dangerous and possible to lose whole hands if you don't know piranhas very well.
Nice fish. Do you know the locality by any chance?
Thank you very much. Not exactly sure, tbh. I'm thinking it could be from Guyana or Peru.
Im not saying that I have endangered fish though. Just easing your minds. 
