Get copies of the last Sept/Oct Elektor magazine, great series on protocol for comm's between processors. Started in January, but last couple are where it really came together.
Yeah, I've been reading them and they are covering exactly this, but I'm not done with them just yet. The second purpose of this post is once and for all learn how to properly power up devices, how to calculate the current needed, watts, how it behaves on long wires, how to convert it, etc. Like the basic guide. I've been running without learning to walk, and now I've reached the point where I have to really learn.
Also, local wallwart for each board unless you want to send out like 10V and use local switching supplies to power each board efficiently.
Is wallwart an small AC/DC converter? The one you specify is DC/DC according to the datasheet (a simple regulator, but way more expensive than the one I had for my breadboard)
You don't give enough details about your system, like how much power each board is going to consume.
That's what I mean when I say I don't really know how to calculate all those things. I've studied all this, but that was several years ago back in high school!