I kind of go about it like a jeweler. I use a big, lit magnifying glass, and very fine brushes. With art pens, it's very important to be sure that the ink won't run when hit with top coat, which even some "permanent" inks will do. The best things is to primer a small spot of any fig, then ink with the pen, then spray on a little top coat. If it doesn't run, you're in business: prime the whole figure, and have fun (the tiny amount of additional primer and top coat won't conceal any detail).
On a general note, the bigger the figure, the more complicated your designs can be without looking muddy at a distance. Remember that most figures are observed at a distance of 2 1/2 feet or more, so that's the level of detail I aim for.
Stripes are really difficult. I've tried doing multicoloured hair on some figures, and it's very finicky. One of the best painters I know doesn't even bother. He paints the figure a base colour, then uses washes and drybrushing for stripes.