Definitely add them slowly. You may want to start out with a pair of the smaller ones, since they will be cheaper. GF are social fish, so two will be better than one. As for tank aggression, I haven't much experience there. Only once have I had tank mates where there was any size difference and the big fish was the king of the tank in that s/he constantly swam along the front of the tank displaying for me. But did s/he keep the two smaller ones from eating? No.
BTW, I recommend pellets as opposed to flakes. GF feed at all water levels and they will sink slowly, which if you happen to have a GF that prefers feeding mid-level, can only be good for him. Another good food, blanched peas. Take a couple of frozen peas, boil them for a minute and then remove the shells. The boiling is credited with doing one of two things: helping them sink and killing any bacteria that may be on them. Then mince the peas, I used a parsely shredder. You can also do the same with lettuce or spinach and I'm told that a piece of zucchini is a big favorite, which I will try later this summer when ours comes in.
HTH
Val