When I fleshed out a Pinkerton posse I basically used the Texas ranger list and gave them the 'life is cheap' special rule and knocked 1 point off each fighters pluck value. I also removed the special Texas ranger 'head for the hills' rule.
I found that this gave access to decent equipment and also the sort of posse make-up I thought Pinkertons would have. Hired Pinkerton agents, backed up, if necessary, by deputised citizens (more thugs).
Also, remember Pinkertons in this time span (as depicted in Deadwood) were little more than hired thugs, so they're not going to hang around when the shooting starts. :mrgreen: