For free meshing I-DEAS meshes surfaces first then meshes the volume using the surface meshes then it will automatically deletes the surface mesh. For this the surface mesh element size should be set the same as volume element size manually.
you can map mesh a six sided volume w/o meshing surfaces unless you want to mesh surfaces which i do not see a reason? For more than six sided volumes you should combine sides together before meshing, it will ask you which surfaces you want to combine together.
For very complicated multi-faceted volumes you do not have a choice but free mesh which if you use PROPER size and order of 2 and then use element quality checks for refinment you will get ythe same answer as hex elements but there will be more DOF. Another option will be adaptive meshing which requires more insight.
