Before we get started let it be known I'm a diehard PC, so my opinion might be a little skewed, but I don't have anything against Mac. I like my old iPod.
Mac popularity: Not sure what you mean here. I'm a college student, and I don't see them too often, even though my University (and every other one) offers deals on Macs + iPods. Maybe it's because they also offer much better deals on Dells. Maybe it's because I'm an engineering major and Macs don't pack the right punch for our line of work. Maybe it's because I'm a college student and we're all stupid broke. Even still, I hardly ever see them around campus. I still doubt anyone will judge you if you use Skype over iChat, and if you make a bunch of money, I'd spend it on iPod Touch, or save it for a car. Also, I have no idea what you mean by Skype not being as popular as iChat, because everyone I know uses Skype but before now I couldn't have named iChat without a Google search ("It's that Apple thing like Skype... What is it?"). Yahoo even counts iChat as a misspell, but not Skype. They're the same thing anyway, only one you have to pay an extra $500 to use, the icon bounces before you use it, and it's snobbishly exclusive to one platform.
Job: Depending on the State you live in, you may or may not have to get a license for being under 16. As for getting the job, I would find someone I know that has a job, you know, connections. Otherwise. apply -everywhere-, be courteous, and if you get an online job application where you have to fill out one of those surveys about whether you trust people, only answer 5s or 1s or else your application won't even get through. You can usually find solutions online. I know, it's sad that the honest people lose these days, but I filled out a million applications and never understood why I wouldn't hear back from any of them until my boyfriend told me about this.
Mac Affordability: Chances are, most of them can't afford it, their parents can, and that's all they got for Christmas last year. The cheapest Mac laptop is the Macbook at $999, the cheapest Mac period is the mini at $699 + monitor cost, though I'm sire you've done your research here. Minimum wage where I'm at is $7.25 - 10% income tax = $6.525. If you work 14 hours a week, that's $6.525 * 14 = $91.35 / week.
Macbook time = $999 / ($91.35/week) = 10.94 weeks = 3 months assuming you get paid every 2 weeks
Mac mini time = $699 / ($91.35/week) = 7.65 weeks = 2 months
The reason I said 14 hours is because the maximum time you can work per week under the age of 15, according to the US Dept of Labor, is 18 hours a week - 8 hours on non-school days and 3 on school days. I wouldn't advise this however. It would only increase your weekly pay to $117.45 and the Macbook time to 8.5 weeks. A job is great, but don't work your carefree highschool years away.
OS X on PC: Never done it but it can be done. It'll take some work though. I have to say, it's not legit, like Windows for Mac. Apple doesn't want you to use OS X on other hardware because it takes away from their hardware sales/your freedom. Anyway, it's known as the Hackintosh and I would applaud you for getting it right. I included a few links. No idea about the validity of any of them. Not sure why you'd want to downgrade from Win7. There is a way you can have both Windows and OS X on your machine. Again, I've never tried creating a Hackintosh. Have fun :D