So instead of answering the nice gentleman's question, let's get into an argument over which guppy is the biggest of them all. Besides, since when is it important who's the biggest when he's asking about a specific species of fish? Either way, I'll be worried about which is biggest when it gets to the size of a whale shark, which IS the biggest fish.
Bottom line:
Rhoms get big, you can only have one in the home aquarium, unless you divide, and that's risky. They take a long time to grow, so if you don't have the patience, get an adult.
As far as the dividing, and breeding thing goes, you never know, until you try, if I had the room (too many full tanks) and the money, I would surely be trying to figure out HOW to make it work!!
ALL piranhas when small, need something to hide under or behind, simply because EVERYBODY needs a bit of privacy from time to time. Place the tank in a high traffic area of your home, like the living room, kitchen, or computer room, this will get him or her used to having people around it's tank. Also, leave your lights on, until you go to bed, and don't cover the front of the tank, this is counter-productive, and only encourages shyness.
since piranhas live in slow-moving blackwater rivers, some blackwater would be beneficial. Use your disgression on a powerhead. A friend of mine had a powerhead for his rhom, and only when she ate the cord, and it was removed, did she USE her tank space.
I don't even use powerheads for my redbellies, I just hang the filter on the side (instead of the back) and they're more active than I've heard of others' being.
Back to blackwater, you can make the tank water "black" (it really looks more like watered down tea) by adding drift/bogwood, blackwater extract, or peat in your filter.
www.fosterandsmith.com have a substrate for live plants that reproduces blackwater conditions as well.
There are lots of threads as to what to feed, so I'll let you check those out, and form your own opinion, after all, it's your fish, and your money. ^^
As far as substrate goes, if you want a lighter colored fish, you can use medium to light browns and whites, that'll make your fish lighter (if using light gravel, use a dark background) if you want a darker colored fish (and rhoms look best in deep black) then use darker substrate, like dark blues, dark browns, and obviously black. Feel free to test some other colors, and see what happens. Also, like with light sub/dark BG, go dark sub/light BG that way, you can see your fish much better.
Let us know what you decide to go with, and good luck!! ^^
