My uncle has 2 bichons and they are brothers. They get along great. I visit him often with my yorkie and daschund and all 4 get along great!
My mother in law babysits my yorkie sometimes when I go away for a weekend. She has a 20 lb shihtzu! He is very gentle with Triss (5 lbs) and loves to play with her. They sleep together on her bed at night.
My advice:
-let them get used to each other slowly, like someone said
-make sure the older dogs know they are higher in rank (get fed first, get treats first)
When I first got Triss my 5 year old daschund didn't want anything to do with her and avoided her completely. I kept them separated when I wasn't around. After about a month, he started becoming closer to her. Now, Triss is 8 months old and her and the daschund are inseperable. They sleep on the same bed (even though theres 2), they eat out of the same bowl at the same time, and if one leaves with one of us, the other waits at the door. They love each other so much.
I didn't push it and just let their relationship develop naturally.