I'm still a teenager,too.. I had to learn a couple of things:
1. I should listen to what the more experienced fishkeepers tell me, it will prevent lots of trouble.
2. If I don't have the possibilities to keep a fish apropriately, I won't keep it at all.
I recommend you wait until you either have a large (120+) tank and get 5 or so rbp's or you save your money till you can get a smaller serra and keep it in the 55. 1 rbp won't be fun for you and certainly not for the fish.
Why didn't anyone suggest exodons? They are a member of the piranha family and don't run too expensive and they stay small at I believe 6 inches max. I am also a teenager so I am as well limited to money and I believe exodons are one of the cheapest options. I might get a single exodon in my 55 because I'm getting a 75 and I'm selling my turtles today so I'll have some spending money! Not sure what I'm going to get yet.
I only enjoy single reds when they're the size of a dinner plate. The average keeper would get bored if they started with one juvenile and attempted to grow it out
A single would not be cool but they are small fish, maxing out around 3-4" or so, if even that big. You can get a nice school of them in a 40-55g tank. I think they are a nice looking fish. They are scale eaters though so you really can't keep other fish with them as in my experience they will aggressively attack other fish. Never tried scaleless fish with the ones I had years ago.