Hi there,
Your project sounds intriguing, and I believe I can assist you with the character modeling tasks you've outlined. As an experienced character artist with a focus on 3D modeling for 3D printing, I'm confident in my ability to adjust your characters' heads and create new poses to match your reference photos.
In addition to changing the heads and adding helmets, I can ensure that the final output is provided in both OBJ and STL formats, meeting your requirements seamlessly. However, I must mention that the pricing will depend on the complexity of the poses and modeling adjustments needed.
Feel free to take a look at some of my previous work @ https://www.freelancer.com/u/Tracer3d , which showcases my expertise in 3d modeling. If you're interested in discussing further details or have any questions, please don't hesitate to contact me.
Looking forward to the opportunity to work together!