Iron Gall ink is indeed the best to get the fine hairlines that seem hard to get with other inks. The plated G nibs from Nikko, Tachikawa and Zebra are indeed more corrosion resistant than the plain steel ones but they are not the best for producing the thinnest hairlines. So while they will resist corrosion a bit better, they won't give you the finest possible line that one wants from an Iron Gall ink in the first place. The modern Gillott 303 can produce very fine hairlines but is sketchy on quality and doesn't last that long.
Luckily, there are good alternatives to Iron Gall ink for practice if nib life is an issue. I use a 60/40 mixture of good old Noodler's Black & Water for pointed pen with very good results. Pelikan black works well too - no need to add water to that. There are many other fountain pen inks that work well for pointed pen work.
The skin on your ink can be stirred away. That is made up of the oxidized Iron particles in the ink. Very few of them will mix back in the ink, most will settle at the bottom of the bottle. For this reason, it is a good idea to take some of the ink out in a dinky dip or other small airtight container for use and open the bottle only for filling.