I've been going through the series again and it seems to me it's more like each season covers 6 months rather than a year. Think about it, in the seventh season Shane's son couldn't have been more than 3 or 4. According to the shield wikia site, season 1 begins about 4 months after the Barn was formed.
It makes sense that after that much time the Strike team would have deep connections with the drug lieutenant they put in power on the first day of the Barn, taking out his competition and getting a kick-back. Terry was also in the Co-Pilot episode after Aceveda convinced him to join the Strike team to take over once Mackey "fails", but he wasn't actually brought on to the Department of Justice investigation until 4 months later in the pilot episode. So after 4 months, Aceveda gets suspicious enough of this Strike team leader that Gilroy forced on him that he decides to formally investigate.
I'm just guessing and have no idea, but it at least seems plausible that this much could happen in 4 months. Acaveda was said to have little street experience so that could be why he is new to detectives like Mackey and decided not to start a bureaucratic fight over getting him and Shane off the Strike team before trying them out. As for Julien, he was always naive about how the police in Farmington actually do things and it wasn't until season 3 with the seizure program that he starts to wise up. It's a confusing episode and makes no sense if you assume that each season covered about one year of time.