I never had an elong so I can't say much but from what I have read/heard it's just like any other piranha which some can be skittish when first introduced (which would last a long time until they are out of their shell) and some are just bad @ss right off the bat.
All piranha could be like that especially juvi/sub adult fish including serra but large adult serra are usually much less skittish. for a 75g or 90g I would say start with a large one, like a rhom that's at least 8" or bigger. all the ones I had/have are not skittish given the proper care.
Not all of them will act like a puppy dog compare to other fish (ie. oscar) when you approach the tank, some will just stare you down some just don't care much and mind its own business.
a lot of people give up their piranha too quickly and didn't give enough time for them to come out of their shell. you need to make sure that piranha is really the fish you want to keep before you get one, just remember they are not those crazy monsters produced by Hollywood films.