a 18" rhom wil cost over 800$ plus shipping usually
and large pirayas (around 11" will cost you around 500+++each)
you could grow out some small pirayas (still 50-100$) but it would still take multiple years and a rhom would take probably 2 decades from baby to monster
Probably Piraya would be the meanest of the Pygos, as well as the largest. But Rhoms get bigger, and usually have a more agressive temprament!! That beauty above (black Rhom) is a good example!!
also die at even the best of conditions for no apparent reason.
your best choises for large p's-
single fish in tank- large rhombeus, and it is true the large ones cost good money. and don't buy small beause there's not a chance you'll be able to grow one out to good sizes with the 1-2" max a year they grow, and slower as they get older.
for a shoal- caribe- very aggresive and much cheaper than piraya, not as large growing or colorful, but also not as cannabalistic. there very very aggresive and max out at 15" if properly kept
get a caribe shoal... don't get quite as big as piraya but are just as aggressive... and way cheaper. you could fit around 8 in your 7'er for life more if they are small.