What are you vaping the Zenith on and what wattage are you using?
If you plan on turning up the wattage and direct lung vape the .50 ohm then 18Mg is going to be too strong imo. If you plan on keeping the .5 ohm coil for mtl and keeping the wattage down, you should be fine with 18mg.
I’m using an iStick 50w (from when i had a subtank) only using it at around 13-15 watts recommended on the 1.6 coil.
The wife didn’t like me using the subtank because of the clouds i was producing……but i am finding with the Zenith if i turn down the air holes i can get away with it but also i can open right up for a bit of DTL when i fancy or when she is not around.
You would be fine at 15-16 watts at 18mg but if you crank the wattage up higher, I think 18mg will be too strong. I vape both MTL at 12mg and DL at 3mg. Even 6mg was too strong for DL for me. You could give it a try but I think 18mg at the full 50 watts would probably knock me to the ground lol