The word "guy" being used other than with an adjective seems odd to my ears. Is there a rule or explanation as to why that is?
These sentences sound ok with either "guy" or "man" or "boy":
- He's a big guy/man/boy
- That guy/man/boy is looks tired (That being the adjective here)
Whereas these don't. Replacing "guy" with "boy" or "man" seems more natural :
- A guy is coming this way
- One guy is enough to bring the wall down
EDIT: Is it all about familiarity? If you know the man or if at least, it's a specific man, "guy" seems to fit. In the example "A man is coming this way", you presumably don't know the man. In "One man is enough to [do X]", it's not about a specific man. Any man could do X. And more often than not, you'd use some form of an adjective for a known or specific person, which ties back to my feeling about the adjectives..