Nommalorel I see, thank you for the additional explanation.
In your case, it would be better to replace the images used in the template rig (the rig that will be reused for other characters) with ones that have sufficient margins, and then use the same size for the other characters. This will result in using images with large margins, but the margins can be trimmed during the export process based on the mesh hull, so the textures will not become huge.
Actually, in the Spine Runtimes Guide, on the page about skins, this method of creating template images is introduced, and it is one of the common approaches:
https://esotericsoftware.com/spine-runtime-skins#Creating-attachments
Rather than taking a method that inevitably requires mesh readjustment, it is better to use images of the same size so that only minimal vertex position adjustments are necessary.