I assume it'd be a bad idea to run MySQL and Postgres servers on the same Linode 512 at the same time. Or would it? I've got some WordPress sites (low traffic) that need MySQL and I'd also like to run a staging server for Rails apps that require Postgres. Is this going to run out of memory very quickly on a Linode 512?

If they're both configured for sufficiently low memory use, I don't see why it'd be a problem. It would definitely be better in terms of efficiency and performance to use a single server, but there's no reason you can't do two.