Um, I think I may be that poster. I at least know enough to be dangerous and the insulin dependent diabetic scenario is the bane of my existence. Federal law governs here though state law may be even more restrictive as states issue licenses, not the feds. Your MVA/DMV should be able to provide info on this as it relates to your state.
If you want the ugly history on the diabetes issue, I can provide a link. The Reader's Digest version is that you can obtain a waiver to drive
some commercial vehicles. A waiver is not guaranteed and not available for every endorsement. For example, school bus endorsements are not ever eligible for waivers.
http://www.fmcsa.dot.gov/rules-regul...on/medical.htm gives the basics on insulin dependency.
The size of the vehicle is only one consideration. Does the driver in question possess a license with the appropriate class and endorsements for the vehicle they will need to drive? If so, then you could be in hot water by denying them based on their history of diabetes.
That said, if the driver in question is having performance problems and or is taken off driving by a doctor, the game changes. Unless you are the manager or know the full story, you probably aren't privy to that information. Coworkers may think they have the full story but I have yet to find that to be the case.