My Ebony was just 2 when we brought Gypsy home at 8 weeks old......Ebony has the nicest nature, is placid, sociable and extremely well trained (passed her gold KCGC award, does agility, totally loyal etc etc) so i was expecting her to love the new pup as much as we did.......oh no!!!!
For the first two weeks ebony wouldnt even be in the same room as the pup, growled at her and on one occasion snapped at her....i was distraught, even posted stuff on here for help, and was in regular contact with the breeder and had even talked about returning gypsy if after a month it still wasnt working.
I was lucky though as after 2 weeks Ebony suddenly started to play with gypsy and they are now inseperable, it was easy as i had to keep them seperate for most of the time at first and very gently encourage a friendship.
i think the point im trying to make is that however good, easy, obedient etc the first dog is, you may still run into problems which will cause you worry, and a new puppy may not just "slot in".
Gypsy is now 18 weeks and like i said they are now best of buddies, and i absolutely love having two dogs (would even love to add a third).....so if you have the time, patience and room to seperate them periodically for a while then go for it