Like it was already stated before...depends on how much into piranha you are. If you are not totaly into it, a solitary black will get old real quick...and when that happens you should let me know. Solitary serra can go one of two ways: either they are incredibly skittish and will take a very long time if EVER to become less fearful. Or they are awsome right off the bat and chase your finger over the surface of the glass. IMO, the only way pygo's are cool is if you have them in a school over 10 strong.
I've always been more into serra than pygo...but thats my thing.
Any LFS that claims they can get blacks I wouldn't trust until I'd seen it with my own eyes, especially if it is young specimens we're talking about. 9 out of 10 times they always turn up to be spilo's and its very difficult to distinguish the two when they are small.
I'd say with 3 reds you'd want to get them in at least a 55 gal tank in a quick hurry. Or buy a few more and purchase a larger tank. Keep in mind that after 55 gal's...depending on where you are looking the price for the hobbey gets serious. If you were able to score a Rhombeus from this lfs and he's like 2'' give or take...he could make it in a 20L for a while. As an adult, he will need to be housed in nothing less than 100 gallons. But man, an adult Rhom is bad ass!
If you do buy the serra and it turns out to be a spilo (happens alot) and it turns out to be a cool fish you want to keep, pretty sure that they can live in a 55 for life.