Can you add a custom play, pause button to a slide video so when the video appears it does not play?

Instead of a slide video, you could use the "video". that way, users can play and pause it. 

Or, if you want to avoid users being able to scrub the video and need to use the slide video

Move the video later on the timeline. If you want, you can add a button on the screen that jumps to a bookmark that starts right at the slide video.