Yes - a player that is ML ready (this includes players that are under 21 and are going to be absolute studs) should probably be in the higher level of minors to get the best coaching possible. However, if you draft a guy who is 18 years old and once you see his actual ratings, he has a long was to go, putting him in AA will stunt his growth (I know this from experience).
That second piece, I meant to say that 'if he is going to be an absolute stud (86+ overall)', then go ahead and put him in the higher levels of the minors. And yes, there are plenty of guys that are not 86+ that can be studs in the MLB (C, P, 1b, LF, RF). And yes, their individual ratings will dictate that, but I was trying to keep it general.
With few exceptions (mentioned above), a guy should progress every year starting in LowA or Rookie ball. Guys that have big bats and will play C, DH, 1B, LF, RF can be rushed more than guys that play infield. Especially middle infield. But they have to be truly MLB ready by their 2nd or 3rd year. These type of guys will usually be 'big get' prospects (top 12 draft picks, high money IFA signings, 48+ overall pre DITR, high makeup DITR players)
To answer the above question. There are many factors. Does your AA team have a better chance to make the playoffs than your HiA team? YOu want to get true prospects the best chance to play as many games as possible. So, be careful, moving up one of these guys might decrease the chance of your HiA squad not making the playoffs and getting you those extra development games. For each player:
Sal Duran, if you put him in AA, he is only going to get one (maybe two) year end promotion bumps (this year from AA to AAA). Next year he will be AAA and you can give him another year end promotion bump, but then you will be paying a major league min salary two seasons from now. Will he be major league ready by then? My guess is yes. He should get a nice power bump after this season and he should see a nice bump in coachable ratings throughout this season. Also, he is only a LF, so there is no need to keep him down longer to develop his fielding. I wouldn't have a problem with moving him to AA. Just keep in mind the above. If you are moving him to AA and he will be your best player in AA with little to no help, you might want to try to supplement the AA squad (signing value free agents that still have options left) throughout the season so that the AA squad makes the playoffs
Paulo Lopez, I would leave him in HiA, I don't think he will be MLB ready in two seasons. Plus, if you truly plan to put him at 3b, he might need that extra year to develop his fielding. Might need the extra year end promotion for that.
Willie Morlan, definitely want to move him up. He is only a RF and his bat will compete.