You are on the right track. The BioMatch method determines the optimum MOI and Mass for each club. But there is a relationship between optimum MOI and Mass. Thereby it is possible to determine the optimum weight, no matter what the MOI is. If the MOI for say a driver is very large, you may end up having the add a lot of mass to the grip end of the club in order to get to the optimum Mass. In such case, it is advisable to reduce the MOI of the driver by reducing the length of the driver. Then the new actual MOI of the driver can be measured again (or calculated). The application will then yield a lower optimum mass.
It is easier to adjust the overall weight of each club than to adjust the MOI of each club. If one preferred, one could leave the total mass as it is and adjust the length of each club to obtain the optimum MOI. Assuming that the clubs are fitted at the correct length for the golfer, you do not want mess around with the length of the club in order to obtain maximum MOI. One can also adjust MOI by adjusting clubhead mass. I personally do not think that is a good idea as it alters the properties of the clubhead.
Hope this is of some help.