as a rule rhoms are not able to be shoaled. you cannot say whether the rhom you buy will be aggressive or not, however most members will argue that on average rhoms are more aggressive. They can get pretty darn large (about 16-18") but grow very slow. They will grow fast up to about 4-5" and then slow down dramatically. If you have patience and are willing to let the piranha adjust to its environment, then a rhom may be right for you. If you want a shoal of fish then go with reds.