How can I assemble a body from parts in game??

Hello, I'm trying to make a character customization UI in gameplay. I found a lot of tutorials regarding this, but my problem is different. I want to assemble body parts one by one... for example, I can start with a head in the air and putting torso, legs, arms, so on... but the thing is, the body parts are different in size so I cannot use one rig. So that I can combine different gender and age body parts together. It would be great if the completed body can be animated like one body, but that's not my priority.( I also wonder how I can put different sized meshes into one rig, or how to combine different sized rig parts.) Please give me any hints regarding this issue. What I'm thinking about is...I might refer to some assembling games such as making airplanes, rockets.

Thank you in advance and I'll answer any comments in a short time.