After Buster cooks breakfast for the Ghostbusters he shows them a lot of food and a long receipt. He explains that he went grocery shopping. It is highly doubtful that Buster could have gone grocery shopping since, well, he's a ghost.
Egon tells the others they can't use their Proton Packs inside the Firehouse to catch all the ghosts Buster invited out of risk of hitting the main electrical grid and blowing up the Containment Unit. However, the Ghostbusters have used their Proton Packs inside the Firehouse numerous times, such as fighting the Spectral Ghostbusters in "Citizen Ghost (1986)", trapping the Sandman in "Mr. Sandman, Dream Me a Dream (1986)", and the time they met Janine's family in "Janine's Day Off (1987)".