Remember the RBA uses 2 coils. So if you go with the 28G 0.8 ohm coils you have above, the total build with be 0.4 ohm (0.8/2), which is fine. If you got the 24G 0.5 ohm coils, that would be 0.25 ohm, which is also OK, although might very well need a higher wattage to perform to your satisfaction. Just always remember to check the resistance yourself to be sure the numbers they gave you were correct.
One question though. Typically mouth to lung vaping is done at lower power, higher resistance builds with low airflow. Are you sure you mean you want to use it for mouth to lung, as opposed to direct to lung (more vapor, cloud production higher airflow)?