Modify 3D Demonic Hand Model

I have an STL 3D model of a demonic hand with five fingers. I need one finger deleted and the remaining fingers resized while maintaining proportionality to the original model.

Requirements:
- Remove one finger from the model.
- Resize the remaining fingers, keeping proportionality.
- Delete any nails (there is only one)
- Add knuckles like the image
- Finger joints rock like, like the image
- Deliver a coherent, modified STL file.

Ideal Skills and Experience:
- Proficiency in 3D modeling software (especially with STL files).
- Experience with modifications like resizing and blending.
- Attention to detail for maintaining model integrity.