Emigration of pops your national culture only starts if MIL (militancy) > 5. Minority pops start emigrating at MIL > 3. Though this is only true if you have a right wing government, either conservative or reactionary (or fascist). Yes, as soon as the liberals (or anarcho-liberals or socialists) are empowered, the drain stops. This is why you basically never get any immigration from liberal UK in most games.
What you're referring to is usually called 'internal migration', and can serve the purpose of concentrating your pops into one mega-industrialized state. I wrote a
short howto explaining how to proceed. As you mentionned it, a good reason behind this is the capitalist mechanism, eg it works better to promote a few big capis pops in one state than scattering thousands accross an empire, since their income is proportional to their size.
One possible exploit is to migrate many pops to a state rich in province with high life-ratings, like New York in the US and London in GB. High LR means increased pop growth, so that, while playing the UK, you can easily hit the 200M hab mark on mainland GB, without even using immigration, just by force-moving everyone to greater London early in game...
EDIT: about your second question: yes, you will need RGOs to feed you industry, plus garanteeing a local production of most basic goods needed by pops, and thus lowering the overall price pops will pay. For an instance, if you stop producing grain, you will certainly notice a decrease in incom taxes, since everyone has know to buy it form the WM - though the price difference is so small you don't even notice it in a big industrialized state.
