I have two cities in Colonial Age and I use 8 Field Guns almost exclusively. The only exception is if I'm against an army of all Rangers, then I use 1 Grenadier with 7 Rogues. That's the real key unit...Rogues. That is, if you're not using artillery. Rogues are sometimes okay with artillery, but generally not. I would recommend one Rogue Hideout/Den, one Grenadier barracks and one Field Gun camp. Although, after you've gotten a couple of dozen Grenadiers, I would get rid of that and just produce Field Guns and Rogues. The Field Guns can handle almost anything in Colonial, and it's good to start building up your Rogue inventory now for later ages.