Amour up to 2 is able to regain energy to quite tired. Amour of 3 and above suffers from higher rate of exhaustion and unable to recover beyond very tired. Anything above 6 rapidly subcumbs to heat exhaustion and would there after be in a permanent state of exhaustion and unable to recover any further.

I personally use urbies with weps upgrades instead of gallowglasses or highland clansmen. Feudal sgts and mounted sergeants are nice. Spanish Jinetes would be very good as well.

Paivse arbs would tire very quickly in the desert and not suited for large battles or attacks.

Billmen (without armour upgrades) would serve well.