If you know them well enough to find them, but they aren’t your boss, then it’s ok.
I agree that it’s ok, in the sense that you’re not wronging coworkers by sending them a friend request.
It’s just that it may not be advisable. It’s worth thinking about whether you really want to know the things you’ll find out by seeing their feed, and if you really want them seeing yours.
There’s no right or wrong answer to that. It’s ok to friend coworkers, and it’s ok to keep more distance. Both approaches have merits.
It’s also important to understand that a lot of people don’t friend coworkers. If a coworker doesn’t friend you back, there’s a good chance that it’s not personal, and it’s important not to take it as a personal insult.