The American Civil War
This is the Madaxeman.com 10mm ACW page. It's OLD - probably earl'y 2000's at best I'm afraid and so whilst it has links to a non-exhaustive set of pics of 10mm ACW figures from a number of manufacturers not all of them are still around, and the ranges may have been updated. The full photo directory is here. It also has some info on which may be compatible to mix together, and links to a number of large-scale ACW battles fought using 10mm figures
The general consensus among my mates (the sample reviewers) seemed to be:
Brigades made up of Minifigs/Pendraken, and Starfort (Crackerline Plank Road)/MM, and then Pithead alone would be fine. GHQ might mix with Starfort and MM, but they weren't maybe as nice.