It really just depends. In an ideal world, your main residency LORs would know you really well and champion your application. Do you think they got to know you and your abilities well enough during a two week period? For some people the answer could be yes, for others, no. Cardio is not my specialty so vmmv in a specialty other than my own, but in my opinion, a lukewarm to poor letter will tank your chances faster than anything. I think you absolutely need a letter from someone in the specialty you’re pursuing, but they also need to be strong letters. I’d ask them if they’re comfortable writing you a strong positive letter and if they hesitate or say no, find someone else. Hopefully they’d decline writing one if they didn’t feel able to accurately evaluate you, but stranger things have happened.
For your second question, it’s usually fine to ask for a letter from a place you’re considering going to but there are caveats. My program’s residency coordinator would not write letters intended to be delivered to her own program to avoid any question of impropriety or favoritism among the selection committee. She’d write them for you and send them to other schools, but if you were applying to her program you needed to find another reference. Other faculty members on the service but not in a supervisory resident coordinator role would write letters even for applicants applying for their own program. There’s no real “rule” to my knowledge, just what everyone is personally comfortable with.
