@vdiluoffo are you using Beta1 or the current master ?
If using Beta 1, you can define the rmw_implementation to use with rclpy by defining the environment variable RCLPY_IMPLEMENTATION:
export RCLPY_IMPLEMENTATION=rmw_connext_cpp
If using master, there is a single env variable used for both rlcpp and rclpy: RMW_IMPLEMENTATION:
export RMW_IMPLEMENTATION=rmw_connext_cpp
Hope this is what you’re looking for