Hey.. I'm the rules designer so I'll answer the rules best I can, but if you want to see the playtester copy, you can email me at Taylor at gangfightgames.com - that goes for anyone who might be interested.
So, these rules are designed for 25-32mm models. I dont see why they couldnt work for smaller or bigger models - but our testers have only tested with those scales.
There are three settings in the book- Chronicle (fantasy setting), Aeon (sci-fi setting), and First Stike (Modern Setting). These are all encompassed by the Gangfight System - a series of core rules that govern everything. The settings are additional rules.
Being released as a separate PDF is the Campaign Setting Builder - which gives each settings rule a corresponding number that allows you to prioritize rules based on your needs. Thus, something like a shadowrun setting would use the First Strike Ruleset, then the fantasy races from Chronicle and some equipment from Aeon.
These are not rules for a vehicle battle- not unless the vehicles are 28mm scale. That is going to be covered by a future release.
The game revolves around a custom d6 system. The playtesters have been pretty stoked on how it works. It emphasizes risk vs. Reward - the bigger risk you take, the better the pay off is if you succeed your rolls.
We are talking about doing a complete reboot of blackwater gulch using the Gangfight system, but in the meantime, adapting Gangfight to a wild west setting would be relatively easy.
I hope that helps!