Boutique Summer-->?

Yes, it is absolutely possible to leverage a boutique IB position into an MM or BB FT position. In order, I would probably go for the boutique IB, then F500 or consulting (depending on both the firm and what you'd actually be doing there functionwise - for the F500 - if your doing some back office work, I would drop that further down the list for sure) and then Big 4. I am ranking these for FT IB, not B school since you are going for SA and not FT right now. You'll have to work the network as usual, but you'll have some solid experience on your resume (hopefully, assuming you work on actual deals, etc within the boutique).

For B school, it will depend on your FT position and not a SA position. B schools care more about quality of experience, responsibility, leadership, etc, than your particular function or the brand of your employer. Your experience is essentially used to see how long you've been employed, and then ammo for your essays to measure the impact you've made and experiences you've had as a professional.

For now, worry about landing a solid SA position and then look towards FT. Good luck.