I'm afraid I can't be any help, but would love to know how you get on.
I tried to wean DS2 who has reflux at 19 weeks on baby rice. We had explosive and runny mucus poo for 2 days, like you I changed straight away so don't know if it would have caused rash. I perceviered for another 2 days, the diarhea did stop then, but he was just so uncomfortable, don't know if it was beginning to make him constipated. I then stopped. I was thinking of trying again at 23 weeks, but just trying to collect as much advice as I can before I try again.