Virtually anything edible can go into a stir fry. Yes, you can stir fry things like milk, or even cheetos (wouldn't recommend it though). I've never seen stir fried potato chips or Snickers bars, but I don't see why you can't do that :-)
There are some basic rules however:
- all ingredients are cut into identical or similar shape and size. Some people call this the "shape rule". If your ingredients don't look the same, they won't heat the same way;
- quick-cooking ingredients go into the wok last. This should explain itself;
- put in the condiments at the end, while you're still on high heat. If your recipe involves starch-water mixture as a thickening agent, however, that goes in on low heat, or the texture of your stir-fry won't be even. Note that there are (mostly regional) exceptions to this rule.
Other things really vary a lot. Sometimes people add the aromatics at the beginning, sometimes they do it in the middle of cooking. In some dishes, you don't want to burn your chili peppers, but in other cases (such as kung pao chicken), you must slightly burn them. But then it's just a matter of trial and error.
However, stir frying involves cooking ingredients on high heat for a very short time, so your ingredients better cook fast. Of course, there's a solution to this problem: you blanch your ingredients in water or "slow deep fry" your ingredients in oil (in Chinese, this technique is called "going through the oil" 过油). Some people may opt to pan fry the ingredients.
There are eight major Chinese regional cuisines, and stir frying is a basic technique in all of them. However, not all of them use this technique equally frequently. These rules of thumb are for Cantonese stir fries, which is probably what most people in the West think of when they think of stir fry. Hunanese and Szechuan cuisine also use stir frying frequently, and the rules are generally the same.
However, in Shandong (Mandarin, northern China) cuisine, rule 3 above is often violated. Frequently, condiments and aromatics are added to the wok first, producing a thick sauce, and then the ingredients are added. This technique is often called "sauce popping" (jiangbao 酱爆); you can't do it the other way round, or either the sauce won't thicken enough, or your ingredients will overcook. The most famous example of this cooking method is probably Peking shredded pork (jingjiang rousi 京酱肉丝), where a sweet-savoury sauce is prepared in the wok, slightly reduced, and then the (pre-cooked) shredded pork is added to the sauce and stirred thoroughly.