Do you have to have a PhD to be a "research scientist"?
Titles depend on institution. In industry, you can be a "research scientist" with no particular degree. In a university, a "research scientist" may be a catch-all term for scientific employees, or it may be a parallel track with similar prestige to a professorship (with similarly high standards). I do not know what MIT's system is. 

Your job responsibilities will depend on your boss (advisor) far more than your title. If your advisor is on board with you continuing your current research, then great; this will be true regardless of your title. If they are doing you a favor allowing you to remain employed while you apply for PhD programs, then they may expect you to do "lab tech" type work. 

It's certainly a good idea to discuss your title, responsibilities and goals with your advisor before accepting the position. But I wouldn't be too concerned about the title. If this were a permanent position for you, I might agree that "lab tech" would be an unfortunate title, and might have unfortunate implications about your ability to participate in original research (especially since you would have to consider your position if your supervisor moves on). But since this is just a temporary stop-gap until grad school, I would focus this conversation on your responsibilities and goals.