There's a little bit more too this than I had initially thought.
Someone on the chatboards with a little bit more knowledge on this subject than I gave me some additional information on the Mexican Libertad series. It looks like there's a few more varieties than just the 2 that I thought I needed to chase down. The breakdown he provided looks a bit more like:
1982-1989: Small Diameter; Lettered Edge
1990: Small Diameter; Reeded Edge
1991-1995: Small Diameter; Low Relief
1996-1999: Large Diameter; Large Eagle Obverse
2000-Present: Large Diameter; Millenium Obverse
He also pointed out that there's another design struck in 1949, 1979, and 1980 that was only 0.925 but heavier overall. Personally, I'm don't think I'm going to be interested in those. My intent has been to collect only 0.999+ fine silver issues (and there's more than enough of those to keep me busy for a while). This also leaves me with a few issues regarding the earliest Chinese Pandas which range from 0.900 to 0.925.
My confusion mostly came from not having seen many of the coins in hand. Many of the changes are subtle and include changes to the rim, changes to the diameter (which don't show well unless 2 coins are photed together) or changes to lettering. Only the major design change in 1996 had stuck out at me.
Well, more fun to be had; more slots to fill. I decided to go ahead and add the slots for the libertads, even if I won't be able to fill them for a while. I think having a few empty slots is nice for showing that the set isn't finished and is still growing with a purpose and a direction.
I'm attaching a picture of a new 2008 Libertad that I got in the mail just Friday. I'm really excited about this coin. I think it (or the other 2008 that I bought) might have a real shot at an MS69 when they go in for grading. There's a seller on eBay that has some already graded PCGS MS69s, but he wants a silly price for them and they're graded by PCGS. The set is currently 100% NGC and I plan to keep it that way. I don't mind having PCGS coins in my registry, but I do not want them in this set.