BG for SYW?
Accepting that at 1:20 and that, unless you had loads of players, you could only do sections of a major engagement, can you use British Grenadier to do Seven Years War stuff using 18mm figures?

Have you looked at the "Die Kriegskunst" forum on this site? I believe these rules are 1:40 or 1:50, but I suspect they could be re-jigged for 1:20. As for BG, I think they are somewhat period-specific (unless you are refighting small quasi-European F&I actions, such as Quebec or Ste Foy). Because of the large numbers of cavalry and artillery, and the size of the armies, perhaps GdeB might be a better bet.

See next to last post in DK section of this forum "troop scale". Also see http://www.edinburghwargames.com/Die%20kriegskunst.htm for more DK info.