This happened to me. My school counts my entire HPSP package, including tuition, fees, stipend, books, etc. as a scholarship. This comes to about 55k a year, which is going to go up this summer. Our school budget is in the low 60s. There is a rule at my school, which I'm not sure but seems to have some basis in federal law, that a student cannot recieve more scholarship money than the student budget. The school had awarded me a merit scholarship, but had to reduce it to the difference between the student budget and the HPSP package.
Note this is a school rule, not a HPSP rule. HPSP doesn't know about any other money you're getting, and they don't care or adjust your package accordingly. There's a clause about outside aid in the Navy HPSP handbook. You're allowed to accept other aid as long as it is not federal and provided it doesn't commit you to anything that would keep you from the Navy at your scheduled graduation date.
If you're talking need based aid, they would likely factor the HPSP package into your fafsa somehow.
Also, I recieved an additional grant for a research project I did, which was not disbursed from my school. That year I got that I got more total aid than the student budget, because the people who gave me the grant don't have to follow the rule about the student budget. My school however cut their scholarship to me for that year.
Don't know about loans.
Contact the medical school's financial aid office for their exact rules.
Hope this helps. I won't try talk you out of the military. But seriously, if you can't keep all that scholarship money, consider FAP. They're paying people nearly 70k a year now in addition to civilian resident pay, and you get much more freedom about specialty and training location.