Modular 18650 battery pack - using differnet chemistries?
It is highly Not Recommended to mix batteries, of different age, charge conditions, brand, and yes, different chemistry. The difference in the cells will lead to uneven discharging and potentially one charging from the other, or inverting, and charging issues. With lipo cells, this could include a considerable risk of exploding. 

Commercial battery packs will test each cell and carefully chose cells with matching specs to ensure they work well together.

So do what it says on most retail battery packs and Do Not Mix Cells. It is not just marketing hype.