1) Depends on the Pokemon in the gym. Go by strength and weaknesses (and CP of course). For example, if the Pokemon in the gym is Electric, you don't want to use Water or Flying. You want to use Ground.
2) Well, IV wins in the long run but if it's low CP you will need a lot of stardust.
3) If you mean if it recognises types and weaknesses/strengths, then yes, as explained in 1).
4) If your Snorlax has Earthquake as its special move then that would explain why - Ground is weak against Water.
Try googling for "Pokemon Go type strengths and weaknesses".
Also there's a great website that has all the best move sets listed etc, just google for Pokemon Gameinfo.
