I'm a recent Penn 2017 Grad (hurrah hurrah to the red + the blue, best place ever). I was a biology major and premed, finishing the premed reqs for bio majors is pretty easy, as most overlap. Even if you do HSoc it's still pretty doable, just plan ahead and double count things. If you take biochem through the chem department and then fill out the minor form, you'll also get a chem minor just from the premed/bio reqs! I took the 2 semesters bio+lab freshman year, and then did the 2 semesters chem+lab and 2 semesters physics+lab my sophomore year, and then 2 semesters orgo my junior year, and took orgo lab my senior year. I'm taking a gap year, but I also didn't decide on premed/bio for sure until about halfway through college. About 1000 kids every year come into Penn as pre-med, and 200 finish as one, so honestly just let yourself explore fun classes too and maybe you'll find a new passion! If you don't want to take a gap year then you have to take 2 sciences (like how I did chem+physics) for one of the years, but if you end up being a bio major you'll be doing way more than that anyway. You can essentially do the same schedule I did and take your MCAT some time junior year, and then apply that June (you're allowed to do orgo lab after applying aka senior year) I mostly just took a gap year because I didn't want to take the MCAT during school and did it while doing full-time research instead after my junior year, but the orgo on the MCAT is nothing compared to Penn Orgo so you could just self-study if you wanted to take it during the summer after sophomore year. Honestly, don't stress it, Penn doesn't like people to go overboard first year, and I think I only met 1 person who doubled up on sciences their freshman year. It all works out, advisors are helpful usually!
And with regard to committee letter for Penn, it is quite a process; make sure you're on the pre-health listserv your entire time at Penn as they always send informational things out. In the year before you're applying (if no gap year, junior year) you just have to go to a workshop in the fall, then by like March/April you have to have an hour-long advisor interview. Before your interview you'll need to write and upload into a portal 5 to 7 500-word essays, and a couple other documents, such as BCPM courses/gpa delineated, expanded resume, and then on May 31st you'll need to upload your AMCAS app draft. You also have to have all LORs into them by May 15th. Seems like a lot, but it's really just stuff to help you with the activities section, personal statement, secondary essays, etc. and keep you on top of the app.
Sorry this is a lot of info, feel free to message me with more specific questions, but just relax and enjoy your freshman year! I hope you love Penn as much as I did!