I agree, time is normally the best solution, along with helping them interact and paying plenty of attention to your older dog.
When I first brought Silvia home, she only weighed 8oz. and I had two VERY large cats (19lbs & 23lbs). I was sooo worried about how that was going to work. I remember laying Silvia in on the floor in front of me while she was asleep, and letting the cats smell her. Well, she moved just so slightly and within a millisecond, those cats were on their hinds popping her repeatedly with their front paws (they're declawed).
It didn't hurt Silvia, but it scared the crap out of me and I remember just crying because I figured they'd never get along.
Well, it took some time, but they are inseperable now and it's like they've been together forever. So Good luck!