Hi and welcome. I "returned" to vaping this time around with a sub0hm, DL, higher vape device and absolutely hated it. You can indeed use any tank on top of your smok mod as long as it has a 510 connection. If you wind up liking say, the Nautilus 2 (it was my quitting tank. Pretty versatile and inexpensive (although IMO there are better MTL tanks out there) but the N2 is good, inexpensive choice, and my husband is now using my N2 tanks to quit (he says).
As far as nicotine strength in a MTL tank, anywhere from 12--36 is common. The higher you go, the less amount of "vaping" you will need to do. I quit using 18 mg/ml juice (and I was a 3 ppd smoker). It was plenty for me, and I did some chain vaping at the start, which is also totally normal for folks to do. It usually settles down over time and after detoxing off all the other nasty chemicals in cigarettes. 6 mg really might be plenty if you were to continue to DL, but if you want to try MTL, you will need to raise your nic. How much, I'm not sure. It tends to be rather individual. You could try some different strengths in small sizes until you hit on your "right" nicotine level, or you could get a "high" nicotine juice and a 0 nicotine juice, mixing them in various proportions until you hit the correct "strength" for you.
Welcome, and keep asking questions until you feel like you are getting "vape satisfaction." Some dual use in the beginning can be normal and if it happens, keep vaping, and those last few cigarettes will fall away over time. Although there are some vapers who take their "first puff" and never look back to tobacco, as well.
Best of luck, and welcome to ECF.
Anna