There is little difference between a and some when used in a/some guy....
A is the indefinite article, so you are giving an indefinite reference. A guy here means any one guy. You are not saying anything definite about him, such as providing his identity. Thus, you do not expect your listeners to know who he is.
Some before a singular count noun such as guy means that the guy's identity is unknown or unimportant. Or you do not care to identify who the guy is. So a guy and some guy are extremely close in meaning. There are just certain contexts where a native speaker might choose some instead of a, which is used in many more contexts.
One way in which some differs in usage is that some guy can (but does not have to) be used in a flippant or disparaging way. In other words, if you want to refer to an individual in a way that shows that you have a low opinion of him, you can use some. But you would also have to use a flippant or disparaging tone. A neutral tone would carry a neutral stance toward the person, just as the indefinite article a would.