multi-robot simulation using gazebo_ros package [closed]

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I am trying to setup multiple robot simulation in Gazebo 1.9 , Ros Hydro on Ubuntu 12.04. I am using turtlebot_gazebo package as base for this experimental setup. I have referred many previous answer on similar lines for example in 1, 2 and 3. But these answers are almost 2 year old.

I have found current gazebo to be very unstable. I have to launch empty world with same launch file to get GUI otherwise gazebo process dies with error code 133. It may be because of following reasons which are my doubts:

1. with Use of <group> tag to define namespaces. Gazebo either opens with black world or do not start.
2. What is the use of gazebo_namespace parameter. I found Gazebo do not spawn the model for all robot which have this tag defiend.
3. I am using ros_namespace for defining namespaces. Is the only correct method to resolve the names,
4. What performance implication do the multiple instantiation of robots have? What i want to know is how many robots can i instantiate in my machine (4GB RAM, Quad Core processor). This question may be more appropriate for Gazebo Q/A site but many people program robots with ROS as well.

thank you nitin