I think that Yami Yami no mi actually nulifies the effects of devils, so no matter how many devil fruits he eats he can nulify the demon which resides in him and take advantage over the power. In the end I think that he will lose control over that nulification in the final battle and the devils will devour him ;p
We didnt actually see him eat the gura gura no mi to be fair. So anything could have happened there that hasnt been explained. Only reason we think he specifically ate it is because as far as we have been informed thats how you recieve the power. We will just have to spe ulate untill oda decides to reveal how he has done it.