Now, I'm not an artist, but I have a question I think one can answer. It's in regards to this game here: http://www.gamedev.net/topic/662953-seven-stage-game-needs-general-feedback (You don't need to read this, it's just a reference if you want it.)

So, there are some enemies in this game that are purely figments of the player's imagination, most of them metaphorical in nature. I decided that, to help illustrate that they're different, and possibly hint that they're not real, I was thinking of using an art shift, their model and texture would have traits of a different art style from the rest of the game enough for them to look out of place, but retaining enough of the main art style for them to still fit and not be jarring. The first one I imagined for this was a character named War and all of his blind soldiers, and I imagined them being drawn somewhat in the style of a political cartoon. But the big issue is... What base art style could I choose that is distinguishable from other art styles without clashing with them, aren't a pain for the modellers or texture artists to draw and still look good?

(By "look good", here's a few good conditions: The player character and other children should be adorable, the home square should look peaceful and inviting, the sections of each act should evoke the emotions the scenes are meant to create through visual design alone, and the regular enemies should look believable and dangerous.)

I would not try to mix art styles. Finding one good art style is really hard, finding two good art styles is much harder and mixing two good art styles is quite a challenge.

Best to take one art style and use different elements to represent them. Eg. if your basic art style would be black/white cartoon, then use an addtional color ,eg red, to represent the others. There are really lot of possibilities, color, shape, animation, lighting, just ensure to use contrast. A very good example is Ico, the girl (player companion) is almost pure light, the enemies are almost pure shadow.