If you're running in test mode (you called adrequest.addTestDevice with your device ID), then your mediation settings are ignored and only the AdMob test ad is returned. You'd have to make a live request to have mediation kick in, but I'm sure the mediation networks wouldn't like you clicking on their live ads during development. We need to think of some way to enable you to send test requests to the adapters.
You don't need to write any code to fire up Millennial Media or InMobi. You just need to include their ad network SDK and mediation adapter. AdMob Mediation will invoke the adapters on your behalf, which will make calls to Millennial and InMobi SDKs. You will want to test out the smart banner format in particular though. Other networks may not support that format, or their adapters may not have been updated to support that format, in which case their adapter may not work with smart banner requests or return a smaller sized ad. It's up to the ad network.
Thanks,
Eric