I'm the last one to say something wouldn't work. I think everything is possible if done well. I do recommend caution if the niches are not related. Even related, people get distracted easily. But that's where your cleverness comes in. You have to organize it so that you stay in command. If the concept doesn't work for some reason, you can always start small sites for each of your niches, redesign the big site, and sell them all if you want, or hang on to some and sell others.

The mini-sites can be sold off if they are not performing where a large site might have parts that are not doing well but not enough to warrant offering the whole shebang for sale. Just something to think about when setting it up.

Hi, I'm still pretty new to internet marketing, and I have to agree with you: promonting internet marketing products is really hard when you don't have the experience to know what you should talk about. I found it easier to make a site about something that I already know, like a software or household item (plus they have a LOT less competition) which i would promote through amazon.

@different sites vs. pages on one website: I'm not really sure, but so far I think it is easier to have one site for one niche. Your readers won't be distracted as much, it will appear more focused and (as far as my limited experience goes) it is easier to rank with a tightly focused mini-site, for example a small 10 page site/blog on greeting cards versus a giant site promoting anything from greeting cards to shovels. But the best way to find out is to try, right? You could start a "general review type" site ex. myreviews.com and a small niche site greetingcardreviews.com (totally made up now *lol*), post some (different) content on both, make more mini-sites and add a category for each mini-site to the "big" site. (I hope this makes sense) And then see which one gets more sales.
Having more small sites might be a bit more work in the long run but personally I think it will be easier to get rankings and traffic.