Well, considering that terminal damage to a gem body (Pearl getting stabbed) is enough to poof them back into their gem and force them to regenerate, I think a gem’s form is a little more important than a hologram (though I know they use that comparison)
If their form was purely up to them, a gem could just regenerate into some other gem, and I feel like that wouldn’t work. Amethyst took a lot of forms in Reformed, but her appearances all had the same core structure. Any time she tried to change solid, physical features in herself she became unstable.
So I think either “Peridots” naturally have those loosely connected mechanical components, and they can’t willfully change that. Or, “Peridots” have a natural solid body structure, and Homeworld modifies them (in probably horrifying ways) to equip them with bionic technology to be stronger/more efficient.
Considering that her weapon is in her weird detachable fingers, I’m more inclined to think Peridots are just naturally able to disassemble some of their parts.