I have no idea what to expect from Diggins or Johnson and would have to defer to the coaching staff's decisions not to play them.
I am a bit more optimistic about Hawkins's upside and impact than I am about Jackson's, primarily because Jackson has had more time and because Hawkins's weaknesses seem--to me--to be more easily fixed with more playing time; and because imo we need Hawkins's scoring more than we need Jackson's rebounds. As
@navery12 notes though, with Martin's injury they should both have plenty of minutes for at least the next few weeks, and with Martin out we definitely have a need for Jackson's rebounds. I agree that both appear to be playing too fast right now, but I get the sense that that is Jackson's default speed and that he doesn't have another mode. That's why I previously mentioned my concern about him calibrating his speed to be in sync with the rest of the team. He always seems to want to push it just a notch faster than his teammates are ready to handle--especially passes.
I don't know what to say about Gaff except that expecting further improvement from him seems fanciful to me at this point, so I struggle with the decision to give him so many minutes at the expense of Jackson and Hawkins, not to mention Diggins.