I want to make a model for a 3d printing transforming figure, tips needed?

So, I want to make third party Transformers at Shapeways like I saw another man do. I will do the 3d models for them to print and they will be ball-jointed. Here is an idea of what I am talking about . So anyway, I have a few questions: 1. I have to make every single piece a separate model so they can be assembled as ball joints, right? Or can I just put them in a single model with space between so they don't fuse? 2. How to make sure the balls hold tight, but not too tight for rotating?

shapeways should give you tolearances for joints.
But f you make the balls about 0.5mm less diameter than the socket, you should get a good fit.
But check with shapeways - you might get away with a smaller difference.
I think the minimum clearance for sls is about 0.1mm (so 0.2mm diameter difference) - but check first.
Oh yeah and as they use sls - you can actually send the model as a complete unit.