It's better to keep symmetry for the left and right side
I'm not sure what do you mean by
"mirror" attached every muscle to the original side of the skeleton
If you name your objects with prefix like l_ for the left and r_ for the right
then you can use zBuilder to mirror them
import zBuilder.builders.ziva as zva
z = zva.Ziva()
z.retrieve_from_scene_selection()
z.string_replace('^r', 'l')
z.string_replace('_r_', '_l_')
z.build()
Also you can use Ziva Transfer menu to Copy everything from selected object and Paste it to another mesh
There is a gap between your fat object and muscles
Usually we recommend to remove this gap for better result
Common solution is to create inner surface of the fat by simulating it as zCloth with pressure and surface tension in rest position and then combine resulting mesh with outer surface by creating a bridge
There is a tutorial about that process
There are some other useful videos on this channel
Hope this helps = )