Hi,
the warnings are viso2_ros related. It seems not able to synchronize its input topics (are they published?) so odometry cannot be computed. rtabmap node is waiting on images and oometry topic, so if any og them is not published, rtabmap will wait with a warning telling it cannot synchronize the input topics.
But yes, normally you can feed RGB + depth to rtabmap directly. There is a demo with zed doing so:
http://wiki.ros.org/rtabmap_ros/Tutorials/HandHeldMappingcheers,
Mathieu