in guided mode you will have acces to velocity, position (both global and local) and attitude controler.
To use them I recommand to use Mavros. You can see here http://ardupilot.org/dev/docs/ros-sitl.html how to connect mavros and SITL. For a real vehicle, just change the fcu_url for your vehicle.
To send goal to the control you need to use the setpoints commands.
If you just want to send speed, you can use the cmd_vel topic of mavros (beware it is twistStamped) or cmd_vel_unstamped (that work with rqt and ros navstack) See old example here https://m.youtube.com/watch?v=JvAzjPYTl6Y
For the position setpoints look at http://wiki.ros.org/mavros for topics name and messages !
In manual and steering mode you can use rc_override or manual_control message to simulated rc radio inputs. Beware that you need to set SYSID_MYGCS 1 on your rover (rosrun mavros mavparam set SYSID_MYGCS 1 to do it for example).
That should do it! Let me know if it doesn’t work or you have difficulties