What you said about fish growing up together being less aggressive towards one another is certainly true of a lot of fish. I didn't mean to say that you'll definitely have aggression problems, but rather that some of your fish have a tendency to become aggressive. Chinese algae eaters are especially known to start out as docile algae eaters, but later in life can turn into jerks.
Honestly, I wouldn't even begin to know how much water you'd need to keep a school of adult iridescent sharks, but definitely in the multiple thousands of gallons. They're hard to keep as adults because they get just as big as adult koi (three feet or more) but they can't deal with cold water, so can't be kept in outdoor ponds in most parts of the U.S. and certainly nowhere in Canada or the UK. They're also more predatory and much more active. Even for a single fish, I wouldn't keep it in anything less than 720 gallons (8' x 4' x 4') for the long term. I'd be even happier if he had something in the thousands so that he'd have more than a few times his body length in swimming room. If you plan on keeping him long-term, your best option might be some sort of indoor pond. I'll be honest, some of my first fish were a pair of iridescent sharks. I had no idea how big they got. The sign at Petsmart read 6" adult length and recommended a 20 gallon tank! After finding out how big they really get, I returned them right away.