Can a mount being ridden by a Level 3 Fighter (Roughrider archtype) with Armored Charger (Ex) ability gain the benefit of the Fleet feat while carrying a Medium load?
You are faster than most.
Benefit: While you are wearing light or no armor, your base speed increases by 5 feet. You lose the benefits of this feat if you carry a medium or heavy load.
Special: You can take this feat multiple times. The effects stack.
At 3rd level, a roughrider no longer suffers armor check penalties on Ride skill checks. His mount’s speed is not reduced when carrying a medium load or wearing medium barding. This ability replaces Armor Training 1.
Seems to me that you would lose the bonus speed from Fleet. Regardless of the Roughrider ability the mount is still carrying a medium load, so Fleet no longer applies.
Agreed. Fleet is a great feat, but it's pretty straight forward on how it works. If you have medium or heavy armor or have a condition greater than a light load, it doesn't work.
I would allow this to work.
It's clear to me that Fleet is designed to line up with the speed reduction associated with wearing medium/heavy armor. I would therefore allow anything that allows you to move at full speed when wearing medium and/or heavy armor also allow you to have the benefits of the Fleet feat under the same conditions.
What about a Dwarf's Slow and Steady racial trait?