Different battery capacities in series is poor
Any time you have a number of devices wired in series, the current through all the devices must be equal*.
So if you have one &quot;small&quot; cell in a string with &quot;big&quot; cells, once you've discharged to the capacity of the &quot;small&quot; cell it will be completely depleted.  At this point, one of two things will happen: either there's no per-cell protection circuits (which is usuall), and you'll start &quot;charging&quot; that cell in reverse; or there are per-cell protection circuits, and that cell will cut out.
At best, your string of cells is limited to the performance of the &quot;smallest&quot; cell in the string, and if you manage it to not damage that cell, you're limited to that cell's performance.  At worst, the first time that you discharge the string deeply, you'll over-discharge that cell and damage it.
* Kirchhoff's current law, or conservation of charge if you want to get fancy.