Too be quite honest Serra Rohm's grow SUPER slow - from a quater size juvi to an 8" fish it takes 3 or more years no matter what size tank they are in. During this time frame they will hide behind whatever, could be 1-2 years before they start swimming around. Rhom's really aren't very impressive til' after 5 or so years Then they really pack on the size and show their aggressive nature. This is an "in for the long haul fish" as adults of 12" or more are quite expensive. I've had this species in the house for over 20 years, currently raising my 3rd, he's been gorwin' up in 55 gal with the lights off for over 2 years and is still only around 7". These guys are nothing like Pygo's that grow like a bat outta hell, quite the opposite, once again raising them is feed and forget for several years so if you can handle owning an expensive fish you never see - and I do mean NEVER see go for it. You will eventually want to upgrade to 2ft wide tank but as small juvi's you got seemingly forever.
As for Payara - I'd wait til' you got a place of your own place and the tank has been maturing/running for over a year - Exodons are an excellant suggestion, RBP's if you can get them over 6" , the lil' guys tend to be more cannabalistic and less toleranat of water conditions. At best 3 RBP's in 75 gal. Serrasalmus Maculatus aka Gold Piranha are great for beginner's - they got attitude as small as 1-2" and can easily be housed in 75 gal for life and are very tolerant of water conditions even as very small juvi's.