I would think it would work well with a group of 4...maybe even a group of 3 but that would be the outer outer limit. Hoever, Mike brings up a good point. When would you roll 2D6 and when would you roll 1D6. Maybe a better solution would be to limit support. You could do something like this....
The first unit uses the combat CF
The second unit adds support.
The third unit adds support -1.
The forth unit adds support -2 etc.
The smallest amount of support would be 1 so numbers less than 1 are treated as 1.
In the case of the cavalry unit.
the CF is 5 and the support is 3. A group of 6 would be 5 +3+2+1+1+1 for a total of 13. That's an extreme unit.
Light infantry has a CF of 3 and support of 2. So, a unit of 6 would be 3+2+1+1+1+1 for a total of 9. That is a winnable fight if only a slim chance. You could keep the die a 1XD6 as per the rules now or perhaps go to a D10 for more randomness.
Adding the "economies of scale" to the support is not too much of a change and the simple modifiers for flanking etc are more in line without change.
John
.