I agree with andi that you have to be very careful. 'Hey pal' or 'hey fella' will almost always sound rude. People don't say 'hey guy'. 'Hey bro' or 'hey man' might be friendly in some situations, but if I used them it would sound like a joke because it's not my natural way of speaking. Slang is only cool when it's natural. To a girl you could say 'hey baby' or 'hey honey' - but again, only as a joke to a good friend. (They sound like pick-up lines! )

There is no difference between any of these, they are all informal greetings. Personally, I don't like these greetings because I think they can be used to disrespect strangers. If you are speaking to a woman, you could use all of these as gender neutral greetings. If you change them "guy" to "girl", or "man" to "woman" it wouldn't be correct, and you run the risk of being slapped by her.