1) Because of the way production is tiered, the two pops created by splitting a larger one will ALWAYS be more productive and thus produce more for your nation, thus earning more money. As for their needs, there will always be a finite supply of goods; since smaller pops have smaller demands, they may be more likely to be met, but as OHgamer explained no one really knows.
2) Beowulf did a good job explaining that, but just to reiterate the important points:
The more of your population that is capitalist, the more they have to share the 'pot' so to speak, so the less they all make. Whether you have a few large cap pops, or several smaller ones, it's the total ratio to your overall population (who actually make the money) that matters.
Larger pops will make money more quickly than a smaller pop, but are limited in how they can spend it. Caps can only build factories in their own state, so one giant cap in one state only can build at most 1 factory every year or two but can build railroads faster. Five cap pops that total the same size scattered over five states will individually build railroads only about a fifth as fast, but overall will roughly build at the same rate and can industrialize five states, but it may take longer.
The difference is that it will cost more to promote five pops than one, but you end up being at to industrialize at lot more over a longer period of time. So I typically promote small pops as capitalists (2-5K) in my most populous states first, and let them grow from there.
As for luxury good availability, it seems that upper class pops ALWAYS get first dibs. I've have never seen a cap pop of any size not being able to get their max needs if they have the money for the goods and the goods are available, even 100K cap pops (the starting one in New York City can grow to that size and split by the end of the game in RIP due to the increased LR in the province).
