From the readings, I have understood that they're aggressive towards fish that resemble their own kind, i.e. black bodied fish, with red in them. Also, they are very territorial, whether their own kind or other fish. If fish are kept with them, they should avoid crossing paths with them, and they should also be fast enough to get away from them. Smaller fish would not be a good idea to keep with them. I am sorry I didn't address the guppies, as I read over that. I doubt they would be good tank mates, as guppies would definitely be targets of fin/eye nibbling from the Red Tailed.