This x about a billion. The only reason I wear cycling-specific shorts is that they don't have a seam running right down the middle, which would be impossible (for me, anyway) to ride with. That's *all* I want my chamois to do: just be a nice little "no-seam" area on my shorts. I really don't dig this trend of the past few years of shorts that make you feel like you're wearing a loaded diaper. If I needed padding at all (and my saddle is pretty comfy for me, I could probably do without it entirely) I sure don't need it halfway up my stomach and butt, which aren't the areas that are sitting on the saddle.

Shorts marketed to triathletes often have smaller, thinner chamois pads, and are supposedly designed to dry very quickly.

Also, w/r/t your mention of the padding feeling like it's "sliding up" or "bunching up" -- make sure you're not buying shorts that are actually too big; the fit needs to be pretty snug.

If you were comfortable for 50 miles, you might be okay for your century. (I usually wait until about the 70 mile mark to evaluate whether something "new" is going to work out for me over the course of a century.) Good luck and have fun on your century!