I thought the big reveal was that almost all trill could be joined successfully, but since there weren't enough symbionts to go around they pretend that it's only a select few that can?

Ezri did not do just fine. She struggled with it constantly, especially at first. No, the training was necessary. It was the exclusivity that was manufactured due to the extreme scarcity of the symbiotes. As it turned out, almost any Trill could join, but that would cause a massive rise in demand, so the Symbiosis Commission claimed only very few were compatible in order to keep demand suppressed and the symbiotes safe.

Ezri constantly talked about how she would forget her own identity if she didn't consciously reassert it.

I mean, she slept with Worf, which is the one thing she's not supposed to do.

And like I said, there may be a difference in personalities between symbionts in how dominant they are. Plus Dax had lived through 8 pairings and knew Ezri wasn't trained. It could regulate.