Power bank DIY Build (Regarding protection circuits)
If you just have parallel cells i.e the batteries will just be 3.7V, then you can combine different capacity cells without a problem.

When you series cells, they should have the same capacity - and random mix of old batteries may not. Your BMS likely has a limited ability to balance different cells on charge. On discharge the low capacity bank gets over discharged.

But you can test this out by monitoring the charge and discharge for a test and see if they are balanced. If they all come from the same packs they might be. However, some laptop batteries fail because a cell has a lower capacity.