At least at my Battalion (PSU), most of our slots are geared towards the upperclassmen. You'd have to be pretty darn high speed to get those slots. This year we got 4 slots: 1 airborne, 2 air assault, and 1 WHINSEC; excluding the X amount of CULP slots.
And PSU is one of the bigger battalions in the nation; and we got 4 slots - so just imagine how many slots smaler battalions got.
My personal 2 cents, plan on doing CULP more than anything else (summer training related). At least for this year, there were still plenty of CULP slots to go around. Remember that CULP only happens as a cadet; one you commission you will no longer have that opportunity; Airborne and Air Assault School, however, you will have that option even after you commission.
Honestly, don't take this the wrong way - but things change, especially coming from high school and into college. ALOT of people have that mind set of branching infantry and/or serving 20+ years AD. But they get to college, start ROTC, and things change for one reason for another.
Trust me, I was there too. Having been in ROTC for about 1.5 semesters now, my goals/life plans have shifted. I'm going for engineering - specifically to go into small arms development and testing. And to be honest, if I can pull an internship this summer with some one like Magpul, H&K, IWI, Bushmaster, etc - which could very well lead to a job after I graduate, it's a very good possibility that I'll take that job, serve my time in the Guard and whatever happens from there happens. Maybe I might stay in the Guard as a career Guardsman, or I might just do my 8 and turn in the keys.
So I guess, basically if you take anything away from this post - keep an open mind. Don't get tunnel vision. Be flexible; as a future leader of America - a future platoon leader, you may make a plan and it may all go to crap. But what separates a good leader and a great leader isn't necessarily the choice he/she makes, but rather how he/she reacts and responds to that unexpected change. You have to be able to adapt to the situations around you.
Best wishes your MSI year. It will be a great experience/learning curve, trust me on that one!