Heresies are fair game, yes. All of the religions have a large amount of customizability, so people should feel free to make their local versions their own. Even Justinianism can be adapted to local norms, since as long as you're serving the Big Guy in Sacrosanctum, local worship practices and folk beliefs are tolerated by the Clerisy. I'll try to get the Dryadicism info up ASAP. It will be similar in many ways to a religion of the Force from SW.