I'd be interested in the answer to that question, too, Sgian. I've come to understand that to really gain the ability to challenge countries like the UK, France, and Russia, you need lots and lots of people. And I've never been able to do more than quadruple Brazil's population in any of the games I've played. I've played Brazil 4 times now, three times under unmodded Ricky and once under VIP:R + Hotfixes, and the best I've been able to do is get my population up to around 24 mil by 1935. (Brazil starts with 6 mil.)
I have never, in any of those four games, ever reached a point where I had full RGOs in any Province.. The states WITHOUT factories in them have RGOs that are 1/5 or maybe 2/5. The states WITH factories in them have RGOs that are either at 0/5 or 1/5 due to unpromotable slaves.
But I get the impression that it's very possible to fill all those RGOs up with POPs -- if you're someone who, unlike me, actually knows how to play the game well.