Yes, there is. The more capis in a single pop the quicker they earn money, which means that POP of 50,000 capis will build factories more often than 10,000 capis POP.
The most important thing to remember with South American nations is that immigration is the key. To get immigration, you need liberal party in power, with full citizenship and pluralism/secularized, to be a democracy, have decent health care and plurality...
Americas get an immigration bonus so you may see immigration with Con Monarchy or limited citizenship but if you have all of those above you will literally be drowning in immigrants.
One strategy if you have a Presidential Dictatorship or Monarchy is to wait a while before you enact reforms. Raise taxes (even at 70-80%, don't be afraid) and tariffs and save some money. Build one or two factories in states that have largest POPs and turn then into clerks to get research bonus. It is very important not to build high tech factories from day one. Steel factory is a safe bet, they bring profit even if you have to import both coal and iron, and it will help you build railroads as steel is scarce in the world market until 1850's. Continue researching technologies that give you higher production efficiency and build level 2 railroads everywhere. Then when your industrial base is decent (in 1948) put socialist party in power to get +25 plurality and enact some social reforms, most notably health care. After that happens, switch to democracy and because of your high number of Clerks you will most often get liberal government in power which will give you another 25 plurality. Now you have liberal government, 50 or more plurality, decent production efficiency, decent health care and all immigrants your economy can find work for...
Concerning converting your POP's to soldiers - there are two possible reasons why the button is greyed out.
1. You don't have materials needed - hover your mouse pointer over that option it should tell you if you lack some materials (small arms, wine etc...)
2. Your party in power is either Pacifist or Anti Military. In both you can't promote POPs to soldiers... Switch to either Pro Military or Jingoist and you will be able. Don't forget, democracies work wonder with immigration but they have very serious drawbacks - like you can't build more soldiers and you have revolts all over or you're involved in a war, because the party in power is Pacifist or Anti Military. Or you're making most of your money from tariffs and citizens elect a free trade party... That's the beauty of Victoria, the world is so vibrant and alive, full of good choices/bad choices that aren't always recognizable. And did you make a good choice or a bad choice often isn't only up to you. More often than not it depends how things in the rest of the world play out....
P.S. Don't be surprised if it doesn't turn out exactly the way you planned for the first time. You may end up with revolts all over your nation, but you'll get it right in the end. One of the important things is to set up your country as good immigration destination in 1853, when Crimean War starts. That's always a long war, involving several Great Powers, whose POP's will want to emigrate somewhere where they can work in peace. If you put your country as the best possible destination, you're in the green...