Look for Kryo's Hull System mod in the Library, that's all you're going to need. With it, you can mix and match any hull type to create whatever you want. It uses blank hulls which are only a hardpoint which you add fake hulls to. All the hulls in the game are available as jewelry so you can combine them together, and you're not restricted to using small hulls on a small fake hull. They're automatically scaled to the initial blank hull. There's also some offset hard points so you can easily merge two or more hulls together, even create formations of several ships. Or at least the appearance of them. It's a bit hard to explain, but's an awesome system for ship building and is the only mod in the libray with the Stardock seal of approval. Check it out and happy ship building.

PS: There's actualy a sticky thread about it above. Check it out, I highly recommend this mod if you're into ship building.