I'm not sure on the base sizes just yet. The minions will most likely all be 30mm, but the skinwalkers may be 40mm since they're are bulkier. I'll have the prints in hand this week finally so I'll know better when I have them in the flesh
Yes, currently you can have any human from any of the starter gangs fighting together, they don't need to come from the same starter set. The gang sets are more like just a way to package a starter gang together and with the same theme, so all you need is 1 box to start playing. You can use the characters in the box, or use the minis to create your own custom gang from scratch.
And yes the heroic characters have their own special skills that aren't available to custom Heroes. That was mostly just for theme and fun, but when I get around to BWG2.0 I plan to have them all as part of a gang leader skill list that you can choose from when building custom gangs. The pre-made characters will have all the same exact rules as custom guys would, just without the extra work of building them yourself