You can probably find the info in the T265 datasheet. If the detail you seek is not available, you can post your query on the librealsense repo for Intel support.
Mainly based on your reference. If you just want to have a drone up and flying, non-ROS is my recommendation. You can also ask for support more easily on this forum. ROS support is handy when your application has something to do with it.
The image stream can be useful for other applications such as precision landing, target detection and tracking etc. The RPi3 does not have USB3.0 so obtaining the image stream is not very stable and might fault the pipeline sometimes.