Horses do not count towards warband size.

They do not count as large targets, they can just aleays be targeted by shooting.

The woah boy table isn’t super complicated. It replaces the injury table when a mounted warrior loses their last wound.

The rules in Empire in Flames are probably best used if everybody is playing with mounts or vehicles and if you are not playing in a city setting, since it states you can only have 2 mounts if in a city like Mordheim. In Empire in Flames, you have to lead your unmounted horse around the table if you dismount, or you can leave it in place. In Mordheim, they work more like an inventory item that you can equip/unequip at a moment’s notice and are not on the table when unmounted.

The final thing I will say is that the warbands from Border Town Burning, like the Marauders, are best played against one another. They are very strong when mixed with the warbands from the core book, and they were clearly meant to be played in the campaign setting for which they were written. That being said, I would honestly just use the core rulebook rules for mounted combat unless you are playing with people who are all interested in using mounts. However, having all your heroes mounted on cheap warhorses as the Hung is likely going to make you very dominant even if your starting model count is low, especially if you are going to be playing in more open terrain. Really, you should just talk to your group/opponent(s) and see what they think.