I like the idea of the shards choosing their host. As for Ruin choosing Ati, its quite simple. Ruin chose Ati in order to Ruin him. Ruin built Ati up, but set him up for failure, because it wanted to Ruin him. Theres a quote in one of the Mistborn books about Ruin understanding that he could build one thing up to knock two things down. Built Ati up, nearly ruined Scadrial, while ruining Ati. Odium, being the shard of hate, chose someone who is hated by all and hates all.