The overall competitiveness of peds and med/peds is definitely greater than IM, just because there aren't as many spots. I really don't know if med/peds is harder to get vs. straight peds...I would guess yes, because there aren't many med/peds spots, but I'm ONLY guessing.
For IM, board scores are less emphasized than a lot of other specialties, though high board scores are definitely helpful at the more well known academic places (think Hopkins, UC San Francisco, Duke, WashU, etc.). "High" for IM is really anything 230 or above...of course the higher the better. I also know that there are quite a few people in pretty high tiered IM programs who only got 220's or so on the boards...this is if they had other good qualities in their applications like solid 3rd year grades, maybe went to a well known med school, and/or did a lot of leadership activities, etc. I'm guessing that med/peds PD's are probably similiar to IM PD's as far as what they are looking for in an applicant in terms of board scores.
I don't think basic science research would hurt, especially if you say that you want to specialize later, but I don't think it's really necessary in your application. At the place where I did IM, more of the med/peds people were going into primary care vs. the rest of us (IM residents) but definitely some still subspecialized.