If there was such a thing as a Gibbus i would get it too. The fish you may be refering to is Serrsalmus Altispinnis (sp?) which is a very sweet looking Piranha. Maybe some of these will be imported soon if there is a demand. Gibbus is also a common mislabel of gold spilos and a regional rhombeus morph
Actually its more than that. S. gibbus is a name that is still valid but not the species. The name is being applied to some S. rhombeus morphotypes, partly because of a revision by Michel Jegu who was attempting to put this name and reinstate the species. However, new evidence suggest the name may not even stick to this morphotype. Anyone who has seen the portrait of S. gibbus (which I have at OPEFE web site) can see the fish looks more like S. elongatus than any other species.
get a spilo i have heard they are more active than small rhoms if that is what you want .but if you just want a p to keep and grow that will be more impressive in a few years time then get the rhom.
imo when buying a fish especially a piranha and spending good money look at it in the long term even if buying something of size ,i have been guilty of selling fish to accommodate others and probably will again but it is good advise even if i possibly dont use it myself