<fixed default> only applies to newly discovered disks. Changing that with a package only entails including the following in a cfx file
<disk> overwrite
<fixed_default> overwrite
qos_disk_total_size = no
</fixed_default>
</disk>
changing individual disks is harder because you have to know the name of the disk ahead of time.
<disk> overwrite
<alarm> overwrite
<fixed> overwrite
<#> edit
qos_disk_total_size = no
</#>
</fixed>
</alarm>
</disk>
repeat the section
<#> edit
qos_disk_total_size = no
</#>
for all known disks (this is the ugly part). Since you are collecting QOS, you could generate this section with a sql query out of the s_qos_data table.
The "edit" operation will change the key if there but not add it if it is not.
-Garin