The NPR % Strength modifier is still bugged / poorly implemented. If you choose a strength over 100%, somewhere in the NPR planning code that increase gets overlooked for some things, resulting in an NPR starting strong then crashing. It tries to spend 150,000 tons of minerals per year while only actually producing 100,000 tons, or something.
But yes, the NPR strength modifier applies to all NPRs (though please note that starting strengths/tech levels/other points ALSO vary for NPRs, from something like roughly half yours to roughly three times yours), even the ones on Earth in a multi-power start.
If I am remembering correctly, then the same 'Add an Empire' pop-up window that allows you to generate the Empire as an NPR also allows you to specify the starting population (in millions). More than you = tougher than you; less than you = easier.
(If you make absolutely no changes to the starting set-up, Aurora will give you 500 million starting population, so I suppose you could consider that the baseline. But for YOUR game, the 'nominal' amount is YOUR pop.