Can you ever be friends with an ex? Should you?
Only if you can see a friendship working out for both of you. How do you determine that?
You need to know why you're getting into this relationship. Are you doing it because you really just want to be friends with the person, or are you doing it because you still have some lingering feelings of attraction or resentment? If you fall into the latter category, then your relationship is headed for drama. The physical and emotional boundaries inherent in a friendship will be compromised and you-or both of you-will slip back into your comfortable relationship roles. As roles are fundamentally inauthentic and lonely, you'll begin to take your unhappiness out on each other, taking things personally that shouldn't be, and using drama as a tool to reconcile your feelings. I've seen this happen time and time again. I've been both the victim of that drama as well as the perpetrator and it's exhausting. It interferes with the closure process and will be a major source of instability in your life. Don't do it, even if you want to. Just stay civil without having a friendship.
If, on the other hand, you've put those feelings aside and can both really honor the boundaries set up to protect the friendship, then I leave it up to you. If you think it's worth it and really believe you have the necessary power, then good luck. I know I personally don't have that ability, but I can see how some people might.