Don't make me go dig up my posting explaining haste stacking again.
Both Pact of the Wolf and Celerity are type 1 haste. They do not stack. However, the haste is in different slots, so you -can- have both buffs on you at the same time... but Celerity won't give you any added benefits over Pact.
The short form on haste spell stacking is this:
If you look up the two spells on Lucy and they both have the same haste "version", they won't stack. How do you tell the haste version?
Increase Attack Speed by XX% (Like Celerity and Pact) = V1
Increase Haste v2 by XX% (Like, uhm, Composition of Ervaj) = V2
Increase Haste v3 by XX% (Like Aura of the Muse Effect) = V3
95+% of all haste effects are V1.
V2 haste basically does not exist except for a few gimpy bard songs.
V3 haste is "overhaste" and exists mostly on clickies and bard songs.