How do I form a Collegium of Wicca (or of Asatru, Islam, etc.)?
The simple answer is, you can’t. Strictly speaking, the official Collegia are based on historical Roman models and are specifically involved in the practice of the revival of the historical Religio Romana. While the Senate can officially recognize religious institutions outside of the Religio Romana, they will only do so for historical reconstructionist groups of traditions that were actually present during the classical Roman era (such as the Mithraic Mysteries, the cult of Isis, the cult of Dionysus, etc.). Since Wicca, Asatru, Islam, Amerindian religion, etc. weren’t known to the classical Romans, they really have no place in the public religious life of Nova Roma. This in no way says that citizens cannot practice these faiths in their private religious life; this restriction is strictly applied to “official” public religious expression.