Alright, there are a couple things going on in this thread so I'm just going to ramble off a bunch of info.
A sub-internship is where you are given "intern level" responsibilities, most (all?) schools require you to do one.
Away rotation is just what it sounds like, a rotation done at another insitution, but it is not necessarily a sub-i.
You will have time to do electives before sending out apps. Although it is not an ideal set-up, it is how many places run. Some school do have elective time (one rotation usually) third year and I think that is something to consider when weighing which school to go to.
A word on away rotations. They are valued differently in different fields (I hear surgical fields value them the most, but I'm only an M1 so I'm no expert). If you search through the internal med forums there are debates as to the value and some people even recommend
not doing them. The take home point is you can do 0 away rotations and be fine.
For instance, I'm interested in neuro-related specialties, such as neurology or PM&R and others, but one of the schools I'm accepted to (Jefferson), the neurology rotation isn't until 4th year (as are all the elective rotations).
In this sitation I would take the time to shadow in the different departments early on in med school and research the fields trying to determine which of these specialities you like the most. Hopefully you would have your mind made up by the beginning of fourth year and schedule the rotation in the speciality you want to apply to as the first 4th year rotation.