gościu is sometimes used especially to people you are not friends with
yeah, this is neutral... used sometimes when we refer to someone who we don't know.
Also...
ziom and variations - ziomek, ziomal, ziomalek, ziomas, ziomuś
dude - koleś or stary
typ - used sometimes in situations like
'ej typie, a wpie$#%@ dawno nie dostałeś?',
'ej typie jak cie tu jeszcze zobacze .... '
there are also some variations of koleś - for example koleszka
also non positive word - leszcz, 'spadaj stąd leszczu!'
if someone know equivalent words for Polish slang that I used here then feel free to translate sentences above.
stary ziom"
I would say yes, sometimes, not often used ... it would mean sth like 'old (but current) friend'
google.com/search?q=%22to+m%C3%B3j+stary+ziom%22
'to mój stary ziom' - this is my old friend