In the wild - they have miles of space to get away with each other. So yes, snakes live with other snakes and lizards. But it doesn't work well when you confine them to a 2-ft x 6-ft space. It may seem like they have room to stay away from each other, but it's not that much in comparison to the wild. Even if they don't physically harm each other, there's a chance that the presence of another animal would stress them out, potentially cause them to stop eating, etc.
The smaller stuff may not be a problem, but it wouldn't really add much to the terrarium and seems would be more hassle than it's worth.
My $0.02.