The answer ... is Yes. Trump is no longer an outsider candidate. Trump is POTUS. Those agencies work for him. Trump should take whatever actions are needed to ensure that he can trust the people who work for him. Trump won’t even fire Sessions and Rosenstein.
In due time. Meanwhile, Trump is right to avoid what the left and GOPe/RINO NeverTrumpers would consider a Saturday Night Massacre and the ensuing clamor for impeachment.