The first way is to limit the combinations of equipment units can have so, for example, a bunch of guys with crossbows can't also have mounts, field plate & awesome melee weapons (and again, Dom3 would be the go-to source for good examples).
Wait a second - is army cost a concern in Elemental ? Because this is what I had in mind. Your proposals seem centered about the notion of limiting what player can choose
, which in my opinion defeats the point of having the ability to design units. Instead, I would like the game to provide reasons for variety of choices
. Player should want to make choices, as there would be pros and cons for most pieces of equipment.
To use Dominions 3 as an example - you pay gold for the "lifeform", which includes HP, morale, magic resistance, basic attack, defense, protection, movement. Then you pay resources for the equipment, which usually gives bonuses to attack, defense, protection, weapon length. But almost all items cause fatigue and you can optimise for different stats. There are reasons to optimise. Only magic equipment infused with air magic can be completely weightless.There is also a third, hidden cost - you can recruit masses of cheap units, but provinces have limited food supply and you need magic items like Cauldron of Broth or Endless Bag of Wine to support huge numbers.
Markatas (small monkeys) are 5 gold. A slinger costs 7 gold. Regular warrior goes for 10. Highly trained soldiers with small bonuses (about 20%) cost 12 or 13 gold, this includes barbarians with 2h weapons and longbowmen. Strict secular elite may cost even 25 gold. Horsemen start at 20 gold, so it's about +10. Heavy horsemen about 35, knights - 50, and elephants - 100.
In Dominions 3, there actually are crossbowmen wearing plate hauberks. They are generally not very popular, because they cost about twice resources than those with regular ring mail or leather. So while a heavy crossbowman may cost 20+ resources and 10 gold, a medium one would be 10/10. An unarmored one might cost 10 gold 5 resources (I don't have the game installed right now).
So if you recruit bands of heavily armored crossbowmen, unarmored ones will outnumber them about 4 to 1. Armour is useful, otherwise you can get slaughtered by 2x faster bows or even slingers (Mictlan has curious club/shield/sling infantry great for this purpose). In most cases the job of crossbowmen or bowmen is to deal damage, not to soak it, so you want to have as many of them as possible, optionally with some basic anti-arrow precautions. There are super-heavy infantrymen, from late Man for example, which have plate armour as well as shields, broadswords and crossbows. But not everyone can afford the resource cost. If you added horses to the equation you'd notice that not only is resource cost above 20, but gold cost is also 20 at the very least. So you pay double gold cost for fewer units, and are you sure crossbowmen need that extra mobility ? Flankers would need it much more, to strike at archers, mages and other leaders. A heavy plate without horses may reduce their strategic move 2 -> 1. Full helmet may be nice but it may lower Precision stat, so it's not optimal for archers and mages. And if you pay extra res for melee weapons but you kill everything from distance, you've wasted them.
Barbarians are really good against tin can type enemies, but absolute lack of armour combined with poor morale means even quite small squad of shortbowmen force them to retreat among heavy losses.
Sending heavily armored troops against giants is a waste, because they will mostly kill you in 1 hit anyway. You want big 2-handed weapons and missiles.
If you want an economy option against archers, take infantry with nothing but shield and a decent melee weapon.