I'm partial to the theory that Pluto, Charon and Triton came from the same parent Kuiper belt object (the orbital mechanics of Neptune's moon are very odd).

Titan is a really weird planet. Look at the atmospheric scale height for Titan. With Titan there is a low gravitational parameter coupled with a remarkably dense atmosphere. It's a perfect planet for doing aerocapture (provided the CN problem doesn't get out of hand).

Titan almost certainly has a rocky core. However there's a lot of funky stuff around that core, e.g. ice, tar, liquid methane, you name it.

Gary