Since slime girls are a versitle liquid creature that can fit any container, would an endoskeleton armor benefit them so they can have a rigid body structure and doesnt wobble all over the place, its like filling them a skeleton so they will have more consistent body
Sentient slimes are just big blobs of gelatinous liquid. As such, they probably weigh a a lot, relative to their volume. And humanoid slimes typically assume that form by conscious choice.
Therefore, any human-size slime person who walks (runs, jumps, fights, plays, etc.) upright on humanoid legs must be either:
1) physically strong in a sense and to a degree that seems impossible, or...
2) magically animated in a way that makes normal physical strength irrelevant.
In either case, if a "leggy" slime could comfortably maintain their humanoid form indefinitely, they'd have little use for an endoskeleton. But if the slime found humaning difficult and/or exhausting, then... Maybe? But I still doubt it.
I say that because you need
muscles to make use of a skeleton. Muscles attach to bones, and the leverage generated by muscular contraction gives meatbodies enough power and structural integrity to move.
Slimes don't have muscles. The mechanics of their coherence and mobility are unknown. In my personal headcanon, slimes aren't even physical beings. Instead, they're magical fields somewhat akin to ghosts. The field of a slime contains and is sustained by liquid. It (the field) is also mobile, and the movements of a slime-field move the liquid it contains. In fantasy fiction, such slime-fields sometimes emanate from a small solid "core" within the creature's larger liquid mass.
SUGGESTION for slimes that need vertical support: enchanted endoskeleton that magically enhances a slime's liquid coherence and mobility in its very immediate vicinity, and which the slime-field can move as though it were liquid. Even without muscles, this would help the slime assume and maintain a roughly human form. A skeleton made of magically frozen, unthawing liquid might be even better, especially with similar coherence/mobility enhancements.
