I'm not sure there's a definite protocol, as long as the register is more or less the same. "Nice to meet you" is a fairly standard, polite greeting, so responding "Yeah, hi" would be inappropriate.
Likewise I agree with, though I find it a bit abrupt.
The pleasure is mine I disagree with. Someone should greet you with "It's a pleasure (to meet you)," before you can respond with "The pleasure is mine."
Nice to meet you, too makes the most sense, to me, or
Nice to meet you
How do you do? could be used as well. It's in a similar register, but could imply that you are not very happy to meet the other person.
These are just personal impressions, though!
GEmatt