There is nothing in the fundamental laws about excluding Catholics. The emperor and the heir had to be Orthodox ... there is no requirement on other dynasts. A mother (with the exception of the empress) did not have to be Orthodox. Grand Duchess Elizabeth and Grand Duchess Marie were Lutheran, allthough Marie converted in 1907. Elizabeth never did, yet her children had dynastic rights, which Nicholas pointed out when Tatiana married.
Thus, non-Orthodox have rights but wouild have to become Orthodox when one became the heir.