main issue with piranha is that a lot of people have the wrong impression that they are blood thirsty killers that will kill everything that enters their water. the fact is that they are often skittish and nothing like what Hollywood presented them in the films they made.
Not that JP needs any confirming for his statements at all but I just wanted to re-iterate this point, my first fish were Red Bellied Piranhas, one thing I will say is think bigger, if you plan for the future now and get a larger tank it is easier in the long run, I started 4 of them off in a 240 litre tank which I recently upgraded to a 600 litre tank. Filtration with Piranhas I find is another big point, due to their common diet it is not something to cut corners on.
Of course, I am a relative moron to most people who browse these forums, but I started with 4 at about the size of a 10 pence piece (a bit smaller than a Quarter?) they are now around 10" long and have all their fins intact.
I don't agree with the statement that you need to start with 40 to end up with 8 or whatever that was said to me as apparent fish experts laughed at me, care from them properly, feed them adequately and they will be fine, I personally love my Piranhas and don't regret starting with them at all.
Well, I hope that helps, sorry for the wall of text and no doubt I have repeated what others have said numerous times before, I just wanted to share my thoughts as I have been down the path you are taking!