There is historical precedence for this. While the Romans employed equites (wealthy horseowners) as a sort of early medium cavalry, most cultures would have lighter javelin cav as their base cav level.

The units you are comparing are for different purposes. The javelin cav on light horse is very effective in skirmishing, but looking at its stats reveals it is a lot weaker in melee. The full charge value of the militia cav is 8 vs. 14 for the Greek cav. And that makes sense as the greek cav are spear armed and on medium horse (the heavier horse will add more as well because of the higher "momentum" stat of the mount.) Neither is going to be particularly effective in melee (although almost all cav units seem to have inflated melee stats.) One thing does stick out, that 4 shield stat for the militia cav. I didn't think any greek cav really used shields at that time? I'll have to look into that a bit. Even despite that, the Greek cav still have an advantage of 2 on defense.

The Equite stats are a bit inflated, they are getting credit for a large shield although they have a small.

As you have said, all the cav upkeep costs are ridiculously low. Cav were very expensive to maintain in the field--keeping them fed in large groups is no mean feat. Any time you read about large cav forces, forage is the primary concern and often limits the size of the cav force, or at least how long it can remain in the field before it must be moved on before the horses begin breaking down to an irrecoverable state. (Upkeep must be easier than an elephant though...which needs 350 to 500 lbs of forage a day! Imagine the associated "clean up" costs.) Upkeep for the horse should probably be based primarily on type: light, medium, heavy, general's bodyguard as well as on whether or not the horse carried substantial armour (cataphract.)