Not to detract from to what you are proposing. But to offer three other options to think about.

1. Is there any way that things could be done on a denarii level as Cran suggested? But say with three levels of division.

For example (just grabbing at numbers):
500 and over – 2 units
200 to 500 – 4 units
under 200 – 6 units max

2. Or another possibility using troop types (cavalry, infantry, range, and spear). With the percentage of your total chosen, directly proportional to the percentage made available for selection.

For example 50% of the available for selection troops for Parthia are mounted. So you could take a total of 10 mounted units.

3. Now if you combine the two ideas above for a third option. That would mean you could take 4 horse archers, but only 2 of any other mounted unit up to the 10 unit limit for mounted Partians.

I have no idea if this would work across the board for all factions, or if my denarii splits are in the right place (and maybe there should be 4 divisions). But I believe whatever is used must be as simple and universal as possible. Otherwise folks won’t use it except in our special contest.

Keep up the good work