To best answer this it would help to know how many teams are in this league and what your spot in the draft is.
If you're in a 10-team league or greater, I would work under the assumption that since there aren't any restrictions on the keepers, almost all of the top 40 guys will be kept and a good number of 41-60 (that where a team's roster versus their keeper philosophy make result in some players outside the top 60 being taken).
I think H. Ramirez, Zimmerman and Greinke are reasonable keeps, though none of them are top 15-20 players. After that, it's pretty much up to your strategy. If you really wanted to grab the top available players in the draft (and again, it would be easier to estimate what kind you might see depending on the number of teams and your draft slot) I agree that you'd have to only keep 4 players (maybe even 3 depending on you draft spot). The other option is to keep all six keepers and decide whether you want to gamble a bit on prospects like Machado and guys with a short track record like Medlen or keep the guys in the 40-60 range like Sabathia and Cespedes.
If it was me, I think Greinke and Sabathia are a sold 1-2 pitching duo and I like Hanley Ramirez and Zimmerman (though both have their own question marks). After that...if you were going to keep two more players I think I'd keep Craig and Cespedes since I think they have upside and could be attractive trade pieces since they've been talked up quite a bit (Medlen is you want to notch a third SP, but I think I'd go with hitters instead). I think Machado should be available in the draft unless someone reaches for him, but he won't last long.
If you go with only 4 keepers, I agree that you might still be able to grab a Craig and/or Cespedes.
It's an interesting dilemma given your current roster, but hopefully we've given you some options to choose from.