I have used spinning gear my whole life and have recently come across some decent casting gear (at no cost to me) and would really like to learn; however, both of the set ups are on 7'6" medium-heavy and heavy rods.  I guess my question is this:  What size, weight and action rod would be best to learn to use casting reels?  These gigantic broom sticks just seem a bit much until I learn to use the reels properly. 

A great beginner bait casting combo is the shimano zeles, its a 6.5 gear ratio right hand reel on a medium heavy, fast action rod. I can't tell you what would be best for you, you have to find that out but I'd definitely recommend that you check it out.

You can learn on the ones you have, bassresource has some vids on how to set them up on YouTube, check them out and practice in the back yard