Each part can be made the correct weight by selecting material on the feature tree.
The COM and CG will not be the same in the assembly since different parts are made of different materials. Example. A Metal tail box vs plastic.
It is a good idea to get the cg correct in the design but usually there is a battery pack that can be moved a little to fine tune the cg too.
The answers you are looking for are all contained within the mass properties function.
On the evaluate toolbar or tab of the command manager is the button I circled called Mass properties. It will pop up the dialog box shown. For any individual model, you can override the mass properties by the circled button. You can override any mass property (actual weight, where the CG is located, moments of inertia, etc.). If you know those values for the detail parts I would use this to set the mass properties for those items. If you check the box "Create Center of Mass feature", you will get an icon in your feature tree and in the model window showing you where the COM or CG (they are the same thing) is. And yes, all of these functions are available in both the part and assembly environments.
Thank you both for your advice, it was very helpful. I did notice though, when I open mass properties and override mass properties, sometimes it doesn't give me the option to insert COM point. I'm not sure why that is.
I do know there will be a battery pack (or two) that will affect final CG of the complete model. Thanks again for your assistance.
I'm not sure how the new COM works, but in older versions you could assign a mass to a part, but that broke the ability to get the COM and moments of inertia. So it was much safer in the older versions to assign a density to your part that would give you the correct mass. If your geometry was correct and your part had a uniform density, you would get the correct values for the COM and moments of inertia.