I know this is an old question but I think even now because knowing how to create an hexapod with plugin that moves the robot around and with an SDF model not a URDF might be useful to someone.

Check this video answer which explaining a bit the process and the result:

So I there wasn’t anything wrong with the model. It must have been something related to the plugin or some strange library error.

I essentially created a gazebo plugin that moves the joints using SetPosition and PIDs and it works just fine. I’m posting here the code I can.

Hope this is useful for you. 

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Answeing the question made in GAzeboAnswers:
http://answers.gazebosim.org/question/12044/hexapod-simulation-not-working/

Here you have some sources used for this micro tutorial:
http://gazebosim.org/tutorials?cat=guided_i&tut=guided_i1
http://osrf-distributions.s3.amazonaws.com/gazebo/api/dev/classgazebo_1_1physics_1_1JointController.html