Can I reuse the skeleton and collision model from one ragdoll to another?

[letmein]

I just want to change the mesh

[Grodbert]

If they have the same size then you'll have no problem in reusing it, if you're planning to make a ragdoll the size can differ a bit with no noticeable differences, but if it's for a playermodel the skeleton has to be exactly the same or hilarity will ensue.

[letmein]

so I have a model that is in .obj format. how do I attach the skeleton and get it into gmod?

[Grodbert]

That's a loaded question, and the actual answer would be longer than I'm willing to type, just import the obj into blender, then select the skeleton with the armature property and export it as an SMD, compile it with your QC et voilà you have the .mdl that you'll need to put into your models folder.