Compatibility between small box and big box vary. Market deck expanders do work with the big boxes (for the most part). Adventure / Variant expanders only work with the core game. Some players have adapted basic Adventure expanders to big box games by using printed card backs and sleeves to match some big box games where the Adventure front motif/attributes is the same as the core game. That may be too much fiddling for some players, but I have seen it work. It does vary depending on big box. For example, it probably wouldn't work with Mist of Zanaga.
Well...after a good night sleep I woke up this morning and still want this game. I even asked myself why I didn't bought it earlier...I got many games since I reentered the BG world 2.5 years ago and since day 1 Runebound is on my list...but I don't have it yet, weird.
I'll get it I promised but I fear a 3rd edition will be announced soon, dunno why and maybe i'm completely wrong. But at the same time, waiting too long could mean I won't be able to buy the game anymore.
*Thinking out loud ON* Maybe I should go to a LGS during lunch...*Thinking out loud OFF*