I dont know how experanced you are with piranhas, so this is just my opion. I think you'd be better off with the reds.. the spilos will be alot more aggressive towards each other and may end up with just one, they must be fed twice a day, and have alot of room as well as hiding spots.. like said they also grow quick.. if your gonna do it stick with 2-3 untill the big tank is done.. Also keep guppies and or feeders in the tank with them as dithers..
MAD