I think titan is a special case. Why? Jupiter has a large moon system that easily fits co-accretion. They have similar sizes and masses and a clear trend in density. But titan is a massive moon with no other saturnian moon to match it.http://www.space.com/11604-saturn-moon-titan-impacts-atmosphere.html http://www.natureworldnews.com/articles/3714/20130829/new-analysis-titan-challenges-current-theories-regarding-saturns-largest-moon.htm I shall stop nitpicking. Triton did completely wreck Neptune's pre-capture moon system, and you are right. Even without such disruption, moon systems can be a mess. Look at Uranus(don't joke)! Most of its moons will collide with each other and most of the irregular inner moons are spiraling inwards. Also, there is another way small moons can form: Collisional disruption of a parent body. There is a reason there are moon families. This does not apply frequently to large moons, but most retrograde moons orbiting the gas giants may be the result of collisional disruption. Moon gets captured, moon crosses one moon2's orbits, moon fails as soft landing and gets blown to bits on similar orbits.