I'd prefer to use
Kelpo's system and change the Inventory Management skill to a Horse Management PARTY skill, so the number of Pack/Cart-horses you can have is based on your HM skill, say you can have HM+1 pack-horses/carts.
Kelpo's system:
http://users.evtek.fi/~k0301161/packhorsies6.JPG
http://users.evtek.fi/~k0301161/packhorsies7.JPG
http://users.evtek.fi/~k0301161/packhorsies8.JPG
I'd only suggest that horses get, on average, 2 slots (+1 for Sumpters and Saddle horses, +1 for heavy, -1 for swaybacked, and -2 for lame) and that carts get 6-15+ slots, depending on their size. The largest carts could even be pulled by multiple horses.
Heroes should be limited to 9 slots, which is exactly enough for full armour, a horse, and weapons. NPC's not in your party should not have their Inventory Management skill changed, since it makes sense that an in-town merchant doesn't need carts to keep his wares in.