I would stick with adsense for now unless you aren't trying to grow your traffic any more. More ads tend to clutter up your site and detract from new customers liking the product. If you are comfortable with your current customers and traffic you could always put in another ad stategically somewhere.
You can use other contextual ads with AdSense as long as they don't mimic the look and feel of AdSense ads, or in any way cause confusion as to what is an AdSense ad and what isn't.
Of course, you can put every ad known to man on your site, but with each ad spot vying for eyeballs, the clickthru rate of each ad decreases. It's also been said that Google now factors into their algorithm the number of above-the-fold ads, and the ratio of ads to content on a page.
IMO, the fewer ad placements the better. However, I'm not saying put all your eggs in one basket. If you want to get a little more complicated than just AdSense, sign up for DFP (Google's DoubleClick for Publishers) ad server. You can sign up for a couple of different ad networks, and rotate them in the same ad spot. You can plug in the average CPM you're getting from the other networks, and Google will automatically show AdSense anytime it has a higher-paying ad.