If you're going to have to drop anyone then I wouldn't pick up any of those guys. You're not going to keep them and they won't offer more production over the guys you have. Maybe over Adam Jones.
If you're going to take a chance then my order would be
Jennings offers a great power/speed combo.
Gordon is 2 because of position, getting that speed and runs from SS is nice.
I'm not on Rizzo's jock like everyone else. I think he will be good but no one can expect him to keep up those numbers in that park with that lineup...yuck.
Ackley sounds like he will help in avg but his power hasn't developed yet.
This is for this year though, it would be a different order if it was a full keeper we were talking about. However, since you won't be keeping any of these guys at the end of the year then you're just looking for help now.
20tm dynasty 5x5 w/BB,HLD
5 SP,3 RP