Having just finished Pharm last semester, I can honestly say that I could not imagine trying to take it in a condensed, summer semester! The shear volume of required reading in a regular semester ate up so much of my study time (not to mention prepping for quizzes & tests). I took 6 classes (17 credits) last semester and it was hectic trying to stay on top of it all. I still managed a 4.0, but it was a struggle to do it!
You, and only you, know what you're capable of doing. I would say that Pharm ate up 12 to 16 hours per week of study time. In a condensed semester, it's going to be even greater. So, think long & hard about what you can realistically do. If you're trying to keep your grades competitive, and you don't have much free time, you might want to hold off until Fall. If you've got the time to devote to it, then go for it. Just know that you're going to be working your tail off all summer!!