Is it OK to connect several lead acid cells with different Ah capacities in series? I know it can be done in parallel as long as their nominal voltage is the same.
No, do not connect different capacity batteries in series, because after the lowest A-h capacity battery is discharged, it will be charged in reverse by the other batteries, quickly destroying that, and possibly outgassing dangerous hydrogen. You would also need to charge batteries individually, or the smaller batteries would be overcharged, again, releasing H2.
However, if the batteries are the same voltage and same construction, i.e., all wet plate, or all gel-cell, then put them in parallel and use a boost converter, such as this example. Of course, you need to choose a converter for your voltage and current needs, and remember that current drawn will also increase in proportion to voltage gain.