I'm a believer in painting a figure as if I'm dressing it. I don't do separate heads and such, since I Frankenstein my figures and usually have to do some putty work in the neck and back of the head, so I start with the flesh areas, then the next layer, shirts, and such, then the jacket/trousers, and finally the footgear. If the headgear is molded separately, I'll do it separately, otherwise it's part of the face/head/hair...
As for detail painting, I "went" to the "Shepard Paine School of Figure Painting", meaning that I use the techniques outlined in his books and diorama tip-sheets... I also refrain from painting details that wouldn't be visible in ordinary viewing situations, like looking at a diorama that's sitting on the shelf or table. Instead of painting detail that is only necessary for photographing a 2-inch figure and showing it 8 or 12 inches in height (like in here), I figure the viewer won't be able to see the missing detail(s)... This removes a lot of headaches and contributes to a better blood-pressure rate.
Since the average person can't focus on anything closer than about 5 " to his/her eyes before the object becomes blurry, I paint accordingly. 5 inches, in 1/35th scale, translates to 14 1/2 scale feet, so I don't do anything that wouldn't be visible on a real person that's standing 15 feet away from me in the real world. I don't do the whites of the eyes, really don't do the eyes at all, since my scenes are general outdoors, and people squint outdoors. So, the eyes don't need to be anything more than a couple of black slits. Positioning them equally is answer, and then moving on to the shadows/highlights. This is where technique comes into play...
Regardless of the scale of the figure, blending is key to face-painting. Stabbing/stippling the demarkation line between the colors is the method I use with oils, enamels, and acryillics... I haven't tried the Vallejo paints yet, so I won't talk about them...
I paint the face and hair's base colors in acryilcs ordinarily, and highlights & shadows in oils/enamels. This is so the shadow/highlight colors don't attack the underlying paint. I usually squirt a bit of clear acrilyc varnish over the base colors as well, to give me another layer between the paints. Then it's just a matter blending...
As for using "Army Men" to practice on, that's fine as far as I'm concerned... Whether you're painting those, or a 25.00 resin figure, the technique's the same. You're using shadow and highlight to depict the shapes found, to show by variations in base color the way light falls on the face. Whether those facial features are molded in or not is moot. Finely molded features just make them esaier to see and allow for a variety of expressions of emotion or environment. On a 2-cent "Army Man", those features are still there as well as on the high-dollar figure, you just have to paint them before you can find them.. It's all about light and shadow, in my humble opinion. Otherwise, it'd be impossible to make a 2D drawing look like it has 3 dimensions, no?
So... After all is said and done, what will matter most is the end result, and how fast you reach your desired level of detail is dependant on how much experience you have in painting faces/figures, right? The more you do, the better you will get, plain and simple. Therefore, if you practice on GAMs (Green Army Men) you'll gain experience faster with less expenditure of money, although you'll go through more paint. But who cares about that?
Frankly, I think that the low-quality of the faces on GAMs will do more for you in finding out what works for you and what doesn't work with much less aggrevation. They also have the added bonus of allowing you to practice repositioning and swapping body parts with no loss or damage to your "real" figures, which in turn will allow you to use the "good" figures on dioramas with any story-line you can imagine, making the figures fit the story, rather than getting stuck with the figures dictating what story you can tell...