I'm going to date my good friends ex
Why are you dating your friends ex?
I thought about it and I think we would be good together.
I can understand how you might have feelings for someone that your friend used to be with. You should ask your friend before you start a relationship with their ex. That way you can avoid ruining a friendship.
You think that they wouldn't want me with their ex?
It is possible that they wont. This is why it is important to ask. You need to be prepared to not date the ex if your friend says no in order to preserve the friendship.