They can really run the gamut - some are super aggressive, some are super chill, most fall somewhere in the middle. I am sure that genetics plays a large role in that, but timid is not a word I'd use to describe the ones I've kept. At least not when they were "healthy".
My blue DG zipped up and down the tank walls for... like.... a WEEK! Then he calmed down. He still does it sometimes (I think he sees a reflection or something).... but now he responds just like any other 'starving' FEESHIE when I walk up to the tank.
Still no problem with my DG and his 8 male endler buddies. He doesn't like it when they get too close at feeding time, but they worked out who eats where rather quickly. I also have lots o plants and a makeshift cave made out of mopani in case the endlers feel the need to hide. But everyone is coexisting fine with no aggression.