Hi there. I did a transitional year at an FP program that wasn't associated with IM and jumped into IM after that year was completed. Got 6 months of credit for my year of TY, which is the absolute max you can get joining IM from another specialty. Once I decided to do IM, I purposely scheduled IM rotations - nephrology, cards, ID. Got credit for radiology, ER, and another rotation (but I can't remember which one) as well. It helped that my IM PD requested the max of 6 months, which may have been because I received excellent evaluations after starting IM.
As for "being treated the same" as an IM rotation, my FP inpatient months were probably just as intense as any IM I have done (and didn't have much OB or peds at all those months). At least that's my opinion.
Good luck!