My observations of typical NPR formations is that they average around ~15,000 tons. Spoiler formations can vary, but from the very limited tabulated data I have from some old campaigns it looks like the Precursors are similar, but Rakhas might have formations under 10k tons.
Personally, I consider the balance between total ground force tonnage and number of commanders to be the controlling factor for the player race, which tends to lead to formations in a range from 12,500 tons to 20,000 tons in my games. Any formations above ~5,000 tons or are mechanically about as equally good as larger formations in terms of breakthrough mechanics, in most practical scenarios.
