Note that the only men that will carry anything are peasants, workers and nobles.
Skirmishers are simply too unreliable and drunken to carry anything.
In order to ride, the total riding capacity must cover the weights of all the units that may not ride themselves. The ox may be driven alongside the horse, but will not carry anything when moving so quickly. A stack will ride if there is enough riding capacity to carry all of the non-riders. Otherwise, the stack will walk. In order to fly, the total flying capacity for the stack must cover the weights of all units that can not fly themselves.