I think previous threads have said that you would want roughly 50:50 inf and art, except that you don't want your cannons to have to fight on the front lines so slightly more infantry is better (maybe 3:2 ratio). Cav is only useful for moving diagonally in battle, so never have more than 4. Later on they are even less useful. Only include engineers if you can afford the manpower, and only 1 per group is necessary. Gaurds are probably not worth the cost unless you are short on national pops or rolling in cash.