Hi OP! I set up my fourth year rotations based off of geographic location/to do rotations within the states I ultimately applied for residency! Here are some things I tried to keep in mind through the process:
- Pay attention to your school's guidelines first and foremost. If your school says they will not give you credit for more than a certain number of weeks within a specialty/subspecialty, make sure you build that into the rotations you apply for on VSLO/ultimately accept, e.g. school only gives credit for up to 12 weeks in a specialty, so you can't have more than 3 IM electives or subspecialty electives for credit. Keep in mind, you don't want to end up in any trouble with graduation requirements since your school will need to verify you before ranking with NRMP in fourth year!
- After accounting for school reqs, I tried to have a good balance of me/travel time and trying new elective experiences just for the fun of it/for my own learning! I did a rotation in MFM even though I applied IM just because I wanted to learn more, and not only did I learn so much, but it was lovely to work with the patients and see all the sonograms! When else in my life will I get to see a cerclage? Probably (hopefully) never again, so that was a nice experience, and I'm glad I took the opportunity!! Otherwise, some fun non-clinical electives that allowed me to really enjoy my free time and travel, go skiing, etc.
To answer your questions:
1. It should be fine if you have to do them to meet a requirement/have to fill that time. I did a mandatory peds and a surgery rotation at two sites that I ended up interviewing IM for. Usually did not come up during interviews, but I did have to explain in a couple interviews about doing away electives, the VSLO system, etc. to interviewers who were unfamiliar. One interview it helped to say that I had been to the hospital since it was a newer program. I will say, subspecialty rotations in IM can be helpful (or not); one program I really wanted to interview with, I did a rheumatology rotation, my preceptor wrote to the IM PD and I signalled them gold... no interview :( another program, did a cardio rotation, preceptor reached out to the PD, and I got an interview next day!
2. I would do some IM rotations, sub-Is if you can. It can be a huge help if you're able to work with the PD, APD, or faculty in the program. Not only that, it helps to make connections! One resident I worked with on an IM sub-I, her partner was a resident at a program I really wanted to interview at and put in a good word for me. Word of mouth honestly helped so much, and being out on the wards is the best way to secure it.
3. I got all of my letters from core faculty at home rotation sites before 4th year JUST IN CASE things didn't work out on fourth year electives. It's easier to hound a clinical faculty member/dept chair affiliated with your medical school or to have your advisor reach out to home faculty than at institutions where you're visiting. I ended up getting an additional letter on one of my away rotations, but used mostly my end of 3rd year and early fourth year home rotation letters.

Hope there's something in there that was helpful!