Let me write out the sugestions a bit more:
Right now we have two concepts.
A) how much goes into one unit
How much goes into one army
My suggestion is largely about A). B is I think less dominant, the afromentioned 9 dragon army only kills one army per turn, so one could just slow them down while otherwise taking the 9 dragon empire elsewhere because they have all their eggs in one basket.
For unit sizes, I propose the following:
1: Multiply the caps (3 at start, 4 with cooperation technology etc.) by 10.
A normal human would have unit size 10, so as far as infantry is concerned, you get 3 per unit.
Cavalry could have a unit size of 12.5, so you get 2 at the start, 3 after cooperation 4 at unit cap 50 etc. In general, cavalry would on average be one less than infantry. I do think that this tradeoff is worthwhile.
Black Widows could get similiar unit sizes to cavalry, or perhaps be slightly bigger. Golems could be unit size 20-25, so when you unlock the current Squad size 5 technology, you can form units of 2 golems. Harridans as Golem sized units should also be ok, having 2 in a unit is still pretty far down the tech tree, and these units would occur at a point where normal units outdamage harridans so much its not even funny.
Dragons get unit size 100, you will not ever have a unit of more than 1 dragon. If you want to put 9 1 dragon units (as you could do right now, if you somehow happen to get 9 dragons) in one army, thats your choice.
For summons, I think it would be worthwhile to add squad size upgrades for summons in the summoner tree. Ice elementals are meh, but squads of 3-4 ice elementals in one unit? That could actually hurt someone!
Interface wise, you summon one guy, and then use upgrade squad size until you hit the cap. Design wise, having to different squad size things running paralell to each other (one for the added unit type "summon" and one for everything else) shouldnt be that hard.