.. to use an old computer phrase: GIGO - Garbage In - Garbage Out ...
t.

In answer to your first question, you are limited only by 1) the capabilities of the harmonizer, 2) your own vocal capabilities ... various brands/models have different features/options/presets ... I'm not familiar with the PSR, but the digitech ex has a number of presets, plus you can create your own .... I'm pretty sure you'll get more in-depth answers from some of the more experienced users on this board ...
t.