I think the number of trade goods you can carry should be based solely on the number of horses in your army. (Unless troop count is > than horse count, than use troops to calculate the room for trade goods.)
The speed of the caravan should be something like (1 + 0.Horse Speed) to determine the speed bonus for the player. Use the slowest horse the person has when calculating the bonus IF horses > troop count.
So stumper would give 133% increase to speed, and courser 146% increase to speed. Also factor in horse heirlooms and you could see some interesting little setups for caravans.
Hope this might help you out chadz.