The reason the mesh is "breaking" when posing the figure is probably that you just tried those poses right after leaving the setup room.  Poser actually splits the geometry of your prop up into non-contiguous pieces for each body part group when you leave the Setup room and replaces the original prop geometry; this leaves the new geometry "un-welded", and once you save it to the Figures library and reload it, the new figure will get "welded" and should stop that cracking stuff.

edit: it's usually safer to leave one extra bone past whatever bones your figure calls for, as sometimes Poser can move the endpoint in very unexpected ways if you delete that extra end bone (like forearms in your case).  You also have cases where the "extra" bone is intended to deform parent geometry, like bending the forearm may be intended to deform the upper arm geometry depending on how it's rigged.

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