it is to get a letter probably depends on how determined the applicant-to-be is to get into Starfleet.
Sisko wasn't sure Nog's desire to get into Starfleet was real, so wasn't inclined to write a letter to support his application. After getting to the heart of Nog's motivation, he wrote the letter. Of course, the letter didn't get Nog into the Academy. It seems to have allowed him to take the entrance exam. He wouldn't get in without passing the entrance exam, no matter who wrote the letter.
As for Ro, we really don't know much about her backstory. Once she was out of Cardassian-controlled space, she could end up on Earth as easily as anywhere else, and possibly just ended up networking with the right people.