[Viperian]

Ahh, thanks! Yes, going by the RAW (rules as written), it does only say "horse", not "horse or heavy horse."

That's too bad -- it really makes sense for a cavalier, modeled after the knights of old, to have a heavy horse to support his armored weight in combat.

And thanks for pointing out where to add the advanced creature template in HeroLab. I never would have thought to look there.
Also, the "click here to add class levels or hit dice" link on the Classes tab was very confusing, since it was gray-colored which I thought meant it was disabled. Also, it would be great if HeroLab could change its hyperlink formatting to something that more obviously looks like a hyperlink, rather than just text. Or, if nothing else, add a mouseover effect that displays an underline under each link when you mouse over it. Still a great tool, though!

Also, to answer another part of my own post, it turns out that I didn't factor in the additional -1 to hit for a horse because it is a size Large creature.