Yes, the .5 is sub-ohm (below 1 ohm).
It largely depends on the person, but as an example I was a 2-3 pack a day smoker and managed to quit instantly with 12 mg in an ego + kanger t2 setup..
Like I said it all depends on you and what you can handle as well as what makes you feel okay. I personally would find 18 mg to be way too much if you do open airflow lung hits on the subtank with the .5 coil.
Try it, if it makes you cough really bad and you feel dizzy after a few drags then it's definitely too much and in that case I would recommend switching to the 1.5 coil, lowering the watts and closing off the airflow so you can take mouth-to-lung hits till you get your hands on some lower nic juice.
'Most' people don't go beyond 6 mg for sub-ohm lung hits, but are quite comfortable vaping 12-18 in a mouth-to-lung setup, because sub-ohm open-air drags create much more vapor which results in more nicotine / drag with the same juice.
On the other hand, if you prefer mouth-to-lung hits over lung-hits then like I said before, install the 1.5 coil, turn down the wattage to like 15 or something, close off the airflow to the smaller holes and you're good to go with that juice
Cheers and let us know how it went!