I've used 10% whole wheat flower before. It works fine. Wheat will gelatinize at mash temps so it can be fully converted, so will rye and barley. You have to work out the dough balls. If you mash in below gelatinization temp it dosen't dough up. Corn meal doughs up pretty bad too but it will not fully gelatinize at mash temps.
Everything is better with a beer.