Well, nobody ever said Fern was a good trainer.
I'm sure Fern wanted to break his monotype for a while, and just used the poison gym as an excuse to try and(fail to) make it seem more like a common sense thing than admitting weakness. Either way, Fern's team got worse after breaking the monotype, which kind of says a lot about his team building skills anyway.
Also, what do you mean about Samurott not being the sole exception to Cain's team? I don't remember him adding non-poison types apart from Samurott to his team. When was this?