Is the best month to go Taiwan on end of July and take my weddding photo ?

I don't think so. It will be during the rainy season, so the weather condition will have negative influence on the photo shooting. I suggest you go and take your wedding photos around April, when it is clear and fine.

How about Oct ?

Hi, Oct. can be better compared with July. The sunny days would be more than that in July. However, it will be still during the typhoon season and rain may fall sometimes. All in all, before taking pictures, I suggest you pay attention to the weather forecast in advance.