OK, here is a solution that will help all out. OP, ask how often they can stock piranha. If it is regularly, or to order, you have nothing to worry about. If it isn't so regular, ask to resurve the number of piranah you wish to purchase. If the later is the case, be prepaired to pay in advance of collection, and order your tank while you are at it. This will make keeping the piranha for you a commercially viable prospect for the shop, but give you time to set up you tank before introducing your fish.
The tank needs to be fully cycled prior to adding your new fish. I gave a link to the fishless cycling thread in a post in your other thread, but incase you missed it, click this replacement > [post="113861"]click me[/post]. If you introduce your piranha into an uncycled tank, you will have ammonia and nitrite spikes, that could (and probibly will) wipe out your new fish. Piranhas are naurally agressive sholing fish, so I wouldn't recomend any group size of less than 5. Anything less that that, and the whole group will become shy, loose colouration and become suceptible to disease.
HTH
rabbut