Whatever you do, the lowest capacity group in a series string will limit the capacity of the entire pack to it's capacity. The highest internal resistance group will limit the capability (current delivery) to that capability. They are probably the same group.
You can add cells to any weaker groups to bolster them, just make sure the new cells are the same voltage as the ones you're adding them to to limit current flow across your interconnects as you install them, and keep in mind that any cell that is already causing you a problem is also only going to get worse, even with other cells to bolster it. If the cell has the wrong kind of problem, it could become a fire hazard. Unfortunatley one effect of bolstering a bad group with extra cell(s) is that it can mask a cell that's rapidly going south because now the BMS doesn't shut off from LVC on that group.
Also keep in mind that any pack not made of matched cells will discharge all the cells at different rates, which means they will end up at different voltages, and age at different rates too (the worse they are the faster they will age because they're being pushed harder, which makes them worse and ages them even faster).
Making matched groups (so each group is the same capacity and internal resistance, etc., as all the others) is better than nothing, if you can't do all matched cells.
A BMS with a balancing function (they don't all have one) can make all the cells (groups) the same voltage (balanced), but they are still different characteristics and will still discharge at different rates, and become different on every usage, and require rebalancing on every recharge.
All balancing is intended (or going) to do is keep the pack as usable as possible for as long as possible, by making whatever capacity is available in the weaker groups all be full (it doesn't matter if the other ones are full, as long as they have at least as much available filled capacity as the weakest group).