MAWS offers a set of free wargames rules for the English Civil War. They’re meant for 25mm/28mm figures wehre one figure equals 10 men
Pike And Shot is John Armatys’ set of free wargames rules for fighting battles of the English Civil War. The rules also can be used to resolve battles of other 17th Century Western European conflicts. The rules set appears designed for 15mm figures, but has special instructions for using 25mm figs.
Wargames Rules 1685-1845 is a set of classic WRG rules from 1977. I own a copy, but you can get your own in pdf form at the link.
A Most Honorable Victory is a set of free wargames rules for miniatures games set in 15th and 16th century Japan.
Here’s an English Civil War variant of the Command and Colors rules. You’ll also need the Command and Colors: Napoleonics rules, which are freely available on the GMT Games website.