'Showpost' URL Format?

Is it safe to just have [post_id].html rather then [post_id]-post[post_count].html?

Or would it clash with something?

The [post_count] part is a number of post in a thread that appears at the top right corner of the showpost window, like this:
'Showpost' URL Format?
So if you will not include it, this area will be empty. Also [post_id].html format is probably too general and may conflict with other url formats. It's better to use someting like "post[post_id].html".. if you still want to remove [post_count].

Thanks for that