I guess saying Dawson should have been happy for them was a bit off, I guess I should have said he shouldn't have been as surprised given that it's not the first time Pacey showed interest in Joey (reference Double Date). Even in FTT, Pacey was quite complimentary when speaking of Joey to Dawson, so there should have been some hint there.

You are right that if the tables were turned Pacey would never act that way. The perfect example is in S5 when he says to Joey and she and Dawson never really got their shot. He was kind of letting her off the hook then because really, they did get their shot, Twice. Pacey was just trying to give Joey a guilt free pass which is opposite of what Dawson would/did do.