[Kref]

Any giant nation:

Game mechanic's doesn't allow place a unit "between" two single squares.
So, 4 and 5 sized creatures make formations with the same density of units per square as 6 sized creatures.
It means it is possible to add 1-sized creatures to the squads of 5-sized creatures in a rate of 1:1, 2-sized creatures to 4-sized creatures in a rate 1:1, and 1-sized creatures to a 4-sized creatures in a rate 2:1.
I've just discovered that and didn't tested, also I didn't read this in guides. Testing is needed to discover reasonable mixes for giant nations. I think that it can increase the durability of giants by reducing amounts of blows at them per turn. I suppose small beings with high defence would suit better for such mixes.