Samsung + either AT&T or Verizon is a welded shut phone. You can't install a custom recovery or any ROM not built on the stock one.
Also the S6 and Note 5 don't have SD card slots and the batteries aren't replaceable by the owner - you have to bring them into a shop and pay, or send them to Samsung for about $40 (and who knows how long). The S7 and Note 6 might solve those problems - S6 and Note 5 sales have been miserable and maybe Samsung learned something. (But I don't see them changing the bootloader [ROM and recovery] situation - they caved into the carriers' demands to make a "look as if we're doing something" solution to the "bring your phone to work" problem. It's about the worst solution there is, but we're stuck with it.)