You are free to use TWI static meshes and textures for your maps and meshes but you are not allowed to make money with this conent (create a mappack and then sell it) or use it for any other project with is not related to RO. For example Epic could sue you if you would use UT2k4 textures in RO.
About user created content be polite and ask for permission but if you use it for RO (and it was created for RO) I don't see a problem. I think you could be sued for using some elses content but he would have to prove it was his but simply don't try to piss off people
Also the static mesh catolgue could use an update, the new content is not included (for example Danzig stuff)