For my 18C gaming I use two rulesets, firstly one for Skirmish games (needs D6/D10/D12 – yes I know needs changing to 1dice type if possible), suitable for up to ~90 figs per side.
Skirmish Rules (Lite) Version 4 (PDF)
Secondly for large battles I use Weight of Fire with some minor modifications, I call my version 'Weight of Fire Plus'
Weight of Fire Plus Version 4 Rules (PDF)
Weight of Fire Plus Version 4 play sheet 1 (PDF)
English Civil War
I have my own version of 'Charge Yr Pikes' by Jackson Gamers, originally I had some minor amendments but recently it had a large change to casualty determination, which seem to work better for my unit sizes. I suppose you could use them for other pike and shot wars (eg 30 years war)
English Civil War Version 4 Rules (PDF)
English Civil War Version 4 play sheet 1 (PDF)
A set of simple rules uses for encounters between Saxons and Vikings – they could be used as a basis for similar rulesets for other periods.