rbps will be a good start as they are much cheaper as compare to other piranhas. however, I would recommend you start with some caribas if you can find them. the care would be the same but they are more aggressive in general as compare to rbps and get a little bigger when full grown.
how long do you plan to keep them in 125? if for long term home I'd say 6-8, if only couple years and you'll be moving them to a much larger tank I would say get 12 or even more. it all depends on how long you will be keeping them in the 125g and how big the new tank is going to be.
general rule of thumb is 20g per fish, but if the tank is 100g and up you can over stock a little (if that's the route you end up going) as long as the filtration can keep up then it's fine.
the other approach is have the tank nicely planted and have few less caribas and it'll be somewhat of a show tank.
the other option you can have is go with piraya, care is still the same as nattereri and caribe but they get much bigger; max at 24" as compare to the 12" for the nattereri and the 15-16" for the cariba. with piraya you'll need more gallon per fish as they get much larger. piraya is more expensive too, so unless you have all the extra cash then better of stay with cariba.
the other alternative is a different variant of nattereri, aka, ternetzi. it has a yellow belly instead of red and gets bigger than rbp.
opefe.com is a good place to look up information on them, here are the links for the pygocentrus group
http://opefe.com/piraya.html
http://opefe.com/cariba.html
http://opefe.com/nattereri.html
here's an other link you can find some general care taking information on piranhas.
http://piranha-info.com/default.php?lang=en&id=page_1
