My experience is that they are definitely aggressive when they are in too small of a tank.
I kept one for a couple years she passed away when she was about 8+ years old, and she was always cruising around dominating the bottom of the tank, but she was never mean. I made sure there was swimming room around the perimeter of the tank and also a couple areas where she could swim in a figure 8. The tank has a lot of plants, rock and DW.
Presently I have a Rainbow which I understood to be less aggressive, but now at a young age and about 5" he seems to be a little more "in your face" to the other fish. I have recently started keeping an eye on him more, and he may need to rehomed to a more semi-aggressive tank than mine.
I think it is their personality. My old RTS was one of my very favorite fish, so I am a little disappointed about this guy. A friend of mine asked if I wanted a RTS she has and I might just find out what the personality is of it and make a switch.
Your tank is large enough, my RTS was 7" and big, females are slightly larger. So you want to have a nice size tank.
Hope that helps some. I would make sure if it turns out aggressive, you have a friend or lfs who would take it. Many semi aggressive fish are larger when they mature and people would rather have a larger fish to put in there, so trading it or giving it away shouldn't be a problem, if it gets mean.
Here is a pic, even though it is a bad pic, it shows how big she was, we were getting ready to move her to the big tank from our old house.