I would think that being linked to them and their inhuman urges and such would be destabilizing for a non-monster consciousness such as Cas' (or anybody else's).
And he was destabilised. But the souls were very different, and the situation was very different, and the results were very different so...I don't get what the problem was. I don't get what it had to do with Sam.
As for Eve's plan--she creates monsters and the souls come to her. Whether or not these souls are created by changing a human soul into a monster soul (not pasted as a fun time, but also not as bad as Luci hatebanging), or by evacuating the body of a human soul and turning it into a monster which grows a freaky monster soul--we don't know, and I doubt Sera is thinking about that.
What I saw happen to Cas was that an angel had a sudden influx of a huge number of powerful, rowdy souls who are bent on evil. He thinks he can do good with the power up gets from all of this, but he doesn't bargain on the sheer taint of the monsterness of the power to make his seemingly kind ideas come out cold, hard, and deadly.
But still able to wander around, because they're not insane. Sam is one soul, a distillation of pure torture in a cage where humans weren't meant to go, the person Luci hates....well, he's on his top 5, anyway. So the vampire soul is able to tear around Purgatory and cause whatever chaos he wanted to, seemingly--no tales of them being tied down and tortured like hell, although I wouldn't want to build a summer home there...
Versus the best torture job *ever*, No competition, no matter how many energy cells Cas swallowed. They're just nothing compared to Lucifer's finest work.