"Old Republic era" is extremely vague. You do realize that covers something like twenty-five thousand years, right?

We recommend looking up a specific period and studying that a bit before you decide when to set things. Its only in the roughly last thousand years of the republic that the Sith numbered a mere two. The Rule of two was set up around 1000 BBY (give or take a few years). 

and what will you do if nobody's interested in playing one of the character ideas you've got? or even two?