Can I use Perform (Act) to act like someone else, replacing a Bluff check?
The description of the Bluff skill describes the set of things for which you use the Bluff skill. The description of the Perform skill describes the set of things for which you use the Perform skill. A quick look through the lists suggests they don't overlap, and Perform certainly doesn't mention anything about replacing Bluff checks with Perform (act).
The cases where you can replace one skill check with another are explicitly spelled out in the rules. For example, you can use the Social Recovery skill trick to replace a Diplomacy check with a Bluff check under some circumstances. Without a rule telling you so, there's no reason to suppose a similar replacement is allowed for Bluff and Perform (act)
It would be reasonable for a DM to rule that having ranks in Perform (act) could provide synergy or circumstance bonuses to Bluff or Disguise checks that involved acting, but this would be a pure houserule.