To shoot slow mo you will have to do 720. We have done it a lot and speaking as an editor you’ll be fine with 720 even though it isn’t 1080.
I don’t see a problem with switching from stills to video. It doesn’t take a lot to rotate the dial. On my T2i I can take a photo while recording video. It of course ruins your video which is why I would just stop recording and rotate the dial.
Hope this helps and best of luck.
Stephen Smith
Utah Video Productions
Check out my Vimeo page