Yes they can as long as the ad units don't look exactly the same or similar. Make them look different and you shouldn't have a problem. I would recommend that you do not overdue your ads though.
We've been testing both Adbrite and Adsense on a few of our client's sites and, our results (your mileage may vary) show higher ctr for both with adsense running horizontal, beneath header/above article and adbrite running sidebar.
Yep, it's perfectly allowed and advisable (as long as the competing ad formats don't look the same). I'm using AdBrite Inlines alongside AdSense - double the revenue stream and, in my experience, definitely a winning combination!