Looking for a mod to disable gain and loss of fat and muscle

You can use MCCC to do this in a couple of ways.

Use MCCC by clicking on the sim you want to lock and in the SIM Flags/MC CAS section you can mark a sim with Flag Freeze Physique and Flag Freeze Appearance and they will never change.

or

Us MCCC by clicking on the computer and in the MC CAS section you can adjust the male and female Fit/Fat limits so that they don't vary. You need to enable the Monitor Physique setting to have the limits you set be active.