It doesn't seem like there's an aggression issue with Riley at all from the picture on the couch...and yes, he is a sweetie. I am still of the opinion that if you make sure that you are solidly the alpha in the house and as long as they both recognize it, there shouldn't be a problem since he's so young. My guess is that he probably WON'T have an issue with Riley since she was there first. I would probably find a more open minded trainer myself. Your trainer sounds like she's anti-Pit, just from your comments. A good trainer should be able to train at least some of that aggression out of him. I agree with Aly, that he should be crated when you're not home. Good luck...don't get rid of him just because someone else doesn't seem to like him. You'll be fine.