I think there are variations in fish personalities just like there are variations within dog breeds, ferrets, snakes species, etc. For instance, reticulated pythons are known to be mean as hell, but I know a couple people that have 20-foot adults that are like big puppies and have never bitten or struck out. It just depends. It may be the minority, but it is possible.
I also have a GSP that has grown half an inch since I bought him, and he doesn't bother any of my inverts (red knob sea star, blood fire shrimp, anemone crabs, snails, etc) nor does he bother any of my other current fish (2 gold striped maroon clowns, 2 yellowtail damsels, 1 blue velvet damsel, 1 blue mandarin dragonet). I think there are 2 important things in keeping them peaceful -- the first is to make sure they are fed enough. GSP's are ravenous eaters with constant appetites, so if they aren't being fed enough, they will start taking bites out of inverts or tankmates or showing aggression because they feel they have to defend whatever food is available. Mine eats twice per day along with my other fish, the first meal being cubes of brine shrimp, mysis shrimp, and krill and the second meal being marine flakes. If I see him looking particularly hungry or starting to pace too much, I give him a small chunk of raw seafood and hold it with a tongs while he takes bites out of it, so that no one else eats it and so that I can remove whatever is left after he's full. Usually this ends up being about every other day and in the evening. The second is to provide stimulation, because GSP's are pretty smart and very curious, so they get bored easily. Providing a lot of rockwork, fake plants, caves, tubes, etc. for them to explore and buzz around will keep them busy and stimulated, and also wear them out so that they are less likely to take their frustration/pent up energy out on tankmates. My little guy is like the energizer bunny, he just spends his whole day buzzing around. At first I didn't have enough stuff in the tank to keep him busy, so he would go up and down chasing his "imaginary friend" (his reflection in the back glass...LOL!). Now that I put in more things to explore, he doesn't do it anymore.
I'm sure the day will come where my GSP just gets so big that he'll inevitably decide to be a jerk to everyone else out of basic dominance and size, so if that happens he'll be moved to a tank on his own. But until then, he seems to be doing just fine. So it's possible to have them work out, but it may just be the exception rather than the rule.
