I hunted a bit with a 20" coach gun, I love the idea of a shorter bbl, but they just don't swing right. Stationary targets, no problem (might as well use a .22 though) moving targets... I like a 20"-24" bbl on a semi or pump and a 26" on a break action.
Yes you can use it, you may suffer a bit on your wing shooting but go for it.
If you do not expect to see any bunnies, I would use 7.5 shot, for bunnies exclusively, I like #5 or #4. #6 is a good compromise for both but I would rather not use it. When hunting with my O/U I carry #7.5 and #5, sometimes #6 and change depending on the terrain, Of course, I will shoot either species should they present themselves, regardless of what I have in the gun.