I spent a few hours trying to sort this out and found a solution. Hope this helps someone else out and saves them some time.
#1 If you have no mods installed:
Enter console command prompt, click on npc, type: setnpcweight xxx. The XXX represents your number between 0 and 100. Save game, done.
#2 If you have a mod for that npc, Seranaholic, Bijin, etc.:
Disable the npc mod in your mod manager, enter game, set npc weight as you would with no mod installed. See #1. Save game. Reenable your mod of choice and start the game. NPC should retain your weight selection previously with the mod activated.
#3 If you want your npc mod to use the custom body that you have activated: (Seranaholic and other overhauls use their own bodies. So if you want the shape and physics of your body mod to effect a certain overhauled npc, do this)
Find the mod in your mod manager folder. Lets use Seranaholic as an example. D:\MO2\MO2\Paths\Skyrim Special Edition\mods\Seranaholic by rxkx22 1.6\meshes\actors\character\Serana. (the beginning of your path will differ depending on where you installed MO2 or if you are using a different mod manager) The files that you want to replace will be here, within the meshes subfolders under the actors name (Serana for this example). They have names like femalebody_0.nif or _1.nif, etc.. Go into your body mod folder, also in a sub folder under meshes (here is an example: D:\MO2\MO2\Paths\Skyrim Special Edition\mods\7B Sevenbase 7Base TankGirl Body HDT CBP and Preset(True Amazon Woman Body) and Muscle Preset\meshes\actors\character\character assets) and copy all of the female....nif files and paste them in the npc mod folder with similar names, when prompted, let your body mods .nif overwrite the npc mods .nif. So I used the 7B .nif to overide the Seranaholic .nif.
Basically, if you want to use a certain body mod on your moded npc, you have to copy the .nif files from the body mod into your npc mod. Then deactivate the mod in you manager, load the game and use the console commands to set the npc's weight. Save the game, exit and then reactivate your npc mod. Reload and your custom npc should use the custom body mod with your custom weight setting.
I tried using ssedit to do this but without success. I suspect there is a way to do this through the creation kit, but I have not taught myself how to use that yet.
***Disclaimer be sure to save a copy of your Skyrim folder somewhere, so that you can revert back to a working copy. I'd also suggest saving a copy of the mod folder somewhere before beginning to edit them manually or with ssedit. If you want, you can try adjusting the weight through SSEdit first, or at least set the values there before following the steps above, but I haven't found that to effect the end results.
If someone knows a better way to do this, please share. Hope this helps somebody else. Its a pretty basic way to adjust the custom follower to your liking and allow them to use the physics of your main body mod.