I think 1% in some pales will be ok. maybe even a bit more. I would just try not to get too many proteins in there, so that it's not cloudy, if that's of any importance to you.
As for belgians... I wouldn't add wheat to a tripel, but maybe a few % in a double. -- but then you have Saisons.. and generally, I tend to be inspired by belgains.. knowing I can never reproduce them, I just go with a "belgian" mind, and see what you have. only cane or corn sugar.. that'll do!- some pale, and some pils left ever.. throw it in there.-- go with what you have on hand, or your heart tells you to throw in there.
http://www.beersmith.com/Grains/Grains/GrainList.htm
willl give you info as to the %s and the flavors assosicated with the grains.