Unless the capris or the liner shorts feel huuuge (i.e. shift around a lot or bunch up around your tush), I would suggest sticking with the mediums you have. To get a good look and feel for the liner shorts, try taking them out of the capris (without cutting the cords in case you want to send them back) and trying them on that way.

I wear size large in Shebeest (Pedal pushers, Shindigger capris, Bellissima jerseys) but with the Boardrider Capris, I had to go up to XL. The large liner short felt fine, but the capri was skintight, so I went up a size in order for the capris to fit well. The waist was a bit loose, but that was actually sort of comfortable. I detached the inner short from the capri. I wear the shorts a bit high on the waist and just let the capris float around my waist, which keeps it from tugging as I move around.

If you ever get regular shorts by Shebeest, though, I'd suggest going down to a size small.

Hope this helps,

~Cari