No, nickel as the metal that is in the soil that the plants are grown in. They just seem to take up more of the nickel than other plants do. Coffee more so than tea. Nickel is the metal that people with metal allergies react to. Or, rather, it is the most common one. It is the least expensive metal that is used in costume jewelry. The coin we call "nickel" actually doesn't contain much nickel at all, just so you know.
Nicotine is an entirely different thing. It is the naturally occurring chemical people talk about when they're talking about tobacco products. It does occur naturally in the tobacco leaves, but tobacco products have loads of other chemicals added to them as well, including additional nicotine.
Yes, and both coffee and tea contain nickel. Coffee has more than tea does. It's not likely to be the caffeine. It's more likely to be the coffee itself.