Welcome to AA!!!
eeeeek!! I don't mean to sound mean but you are definitely in over your head, while river monsters are incredibly smart, beautiful and appealing they are very often wild caught and VERY difficult to maintain.... Not to mention for the stock that you want you are going to need a VERY large river biotope, or at the very least a 200+ gallon tank (think an inch of fish per gallon- and that's adult sized, not juvenile) and with amazon monsters it's even more than that!! One of my favorite riverscapes that my mom has is 700g and includes 3 pacu, a silver arrowana, a small arapaima (my mom wants it gone, though, it killed her gar) and a couple rays- it's an amazing set up, but even with her skills she struggles to maintain that habitat the hardest.
Who in the world sold you those guys, especially with you being so new to the hobby?
It's not fair to you or your livestock!! I would suggest brushing up on your knowledge of Amazon monster fish and requirements and starting slow... With your tank size perhaps it would be a better idea to fully cycle and research in the mean time- if you are bound and determined to keep a piranha, try starting with only one and see where that takes you... Please take my advice and maybe do a google search for piranha care and maintenance and don't overburden yourself and become discouraged.
Good luck!! Be careful!!