Peltier generators series vs parallel
The O.P. has got correct ideas.

"Peltier as a generator" is sometimes called thermopile.  I use this term, because it's shorter.

In series, the voltages will add up.  

In parallel, the currents will add up.  But, a typical load sharing problem can occur.  You might need load sharing resistors.  The two thermopiles wouldn't be dead-on identical.  They would have slightly different impurities, and they will see slightly different temperature gradients.