Well, I was a small animal major who never set foot in "chicken city," but I'll try to answer the basics. Unless they've changed it over the past seven months, "clinical year" at Penn is a bit of a misnomer. A twelve-month year of clinicals was required, but starting in the spring semester of third year, you had the option of skipping additional elective lecture courses and spending either half or all of the semester in clinics. Most people who wanted to do a lot of externships used that time to do it. Since there's only so much room on the in-hospital clinical rotations, third year students entering clinics early are pressured to spend much of that time on electives. You just have to get them approved first. And by "first," I mean months in advance.
As far as core rotations, the required ones were:
Large animal medicine
Large animal surgery
Large animal ECC (all 3 of which focused mostly on horses with the occasional cow, sheep, or goat thrown in)
Small animal soft tissue surgery
Small animal medicine (which was 4 weeks, so twice as long as a normal rotation)
Beyond that, everyone declares a major (SA, LA, Mixed, ect) and that major comes with its own set of requirements. A lot of students found that the mixed animal major was the best fit for people with specialized interests since it had fewer hard-and-fast requirements and left more flexibility to take what you wanted to take.
Good luck going forward. "Two schools to choose between" is a nice problem to have!