The voltage output of the panels is not a problem for series connection. The result is just the sum of all the voltages. After all, the different panel voltages are a result of different numbers of cells in series.
However, the current ratings should match because the same current will be going thru all the panels. Another point that is very important is to put a Schottky diode in reverse accross each panel. That allows the other panels to still push current past a panel that is in the shade. Otherwise, a single panel in the shade will kill the whole output.