For a player collision, you can create whatever Pawn sublcass you want, and add whatever collision object on it you want, no problem! Consider cars, for example.
For a character collision, using the built-in Character movement component, and Character actor subclass, then, no, it’s very strongly tied to using a capsule. It’s not even “physically simulated” in the sense that a typical crate/block would be, it’s more like forward kinematics, so “just add a second collider and make the capsule small” is also unlikely to work.
If you want a fancy simulated humanoid, you probably don’t want to use Character, but instead want to subclass Pawn directly, add your own skinned mesh component and animation blueprint, and add your own physically interacting objects, and write code to drive the skeleton based on the objects (or vice versa.)