Unless you have trades upcoming, do not leave any roster spot empty, except for your rookie team. AAA, AA, High a and Low A leagues starts their regular season at the same time as your ML team. I am guessing that the rookie league season will start AFTER the amateur draft.
Take the projected ratings you see with prudence. This is not exact science. In a trade i am involved in, my projected rating show that player as a 72. His owner sees him as a 87. As he spent $3 millions more on advanced scouting than i did, he might be closer to realty than i do. But , still, there are differences.
When hiring coaches, take a good look at their secondary stats. A pitching coach that also has discipline and patience would be worth it. Do not discount loyalty either. When you get your team, there is a list of the coaches that were there before you took over. I was able to sign 2 coaches that had high loyalty rating even tough my offer was not the highest. One of them responded that he wanted to get in the HOF in my team's uniform. So loyalty might make the job of rehiring coaches you want to rehire a little easier