multiple robots in rviz with different urdfs - robot models not showing

I have a stage simulation with four robots. I wanted to add robot models to the rviz visualization. I was able to add a single urdf for all the robot models easily using:

 <param name="robot_description" command="cat $(arg urdf_file)"/>  in the launch file before the individual robot namespaces were defined. ( https://answers.ros.org/question/4143... ) However when I do the following:  <group ns="$(arg robot_id)">
<!-- set robot model-->

<param name="tf_prefix" value="$(arg robot_id)"/> <arg name="urdf_file" default="$(find multi_rob_sim)/models/$(arg robot_id)_model.urdf"/>  I can't see any of the robotmodels in rviz. I checked if the urdf was loaded using a rosparam get $robot_id/robot_description and it had the right urdfs. My urdfs for now are really simple - (since the robots in stage are just circles too)

 <?xml version="1.0"?>