As Brynglas has stated the istick 20w will not fire under 1ohm so the 0.5ohm coil is out of the question. They are designed for over 20W anyway as well.
The 1.2 ohm coils (which the istick 20 would fire for sure) are still better at above 20w I think so basically it's not ideal.
I see you haven't been
vaping that long (at least from your signature, congrats by the way, hope it works out for you) so I hope you realise you don't NEED to get into sub ohm vaping. I would still (these days) recommend working your way up in terms of power to see what you like.
That being said if you are set on sub ohm vaping then an Istick 30w or 50w would be good value and a better match for a subtank. They have non replaceable internal batteries so while I definitely think it's a good idea to learn about ohms law and battery safety etc etc for vaping, it's not essential for using an istick 30 or 50.
Kanger also do a whole kit for using the subtank called the subox mini I believe, that includes a subtank and a subox, a mod similar to an istick 30 or 50. This does need you to choose an 18650 battery to use in it though and as Brynglas has said you definitely need to do your homework on battery safety and ohms law if you want to go down this route. OR at the very least ask on here or other forums for advice regarding a battery.
We're happy to answer further questions on here by the way, don't be shy.