How do I simulate only part of a object as cloth? Is it possible to make just part of an object cloth? So for example, I've modeled a trench coat, but I only want the loose bottom half of it to be a cloth simulation. Can I do that?
Yes. Use the Pinning vertex group in cloth settings. Any vertices with full weight of 1 will not be affected by the simulation. Partial weight will be partially effected. It is common to use a slight weight gradient for it to work smoothly.