Seriously, just hiring assistants willy nilly without the HC approval? Firing them too?
I think he hired Garrett as OC before Wade was officially hired. He had to do it though if he wanted him, because the Dolphins only granted permission for the Cowboys to interview Garrett as OC/playcaller or HC (and IIRC, they only had a limited timeframe to actually do it).
In the case of Zimmer, Parcells had originally said he was going to hire all new assistants, then agreed to at least talk to Zimmer after Jerry begged Parcells to keep him. Parcells later claimed Jerry never said he HAD to keep Zimmer, but Parcells had been a 3-4 coach all his life and Zimmer had no experience in it. Keeping him made no sense. Two years later Parcells forced the switch to the 3-4 and basically said anyone who didn't like it could leave (he was talking to Zimmer)
The only coach after Jimmy that Jerry really had no say in assistant coaches with was Switzer, but it wasn't because Switzer was in charge. It was only because Jones and everyone else knew the Cowboys had a loaded assistant staff and there was no reason to change anything.