Why connecting different capacity batteries in series should never be done!
Connecting batteries of the same voltage but with different capacities is not recommended. Different capacity batteries will have internal resistance differences, which translates into slight voltage differences, which means the batteries with higher voltage potential will try to charge the battery with lower voltage potential, leading to the lower potential battery being overcharged.
Never connect different capacity batteries in series. The lower-capacity battery will charge first, and the larger-capacity battery will remain under-charged. The lower-capacity battery will overcharge and can overheat. During discharge, the smaller battery will be over-discharged.