1- Any Bayo that fits the Ar-15/M-16 (m7 or m9 bayonets) will fit the 590.
2- Call me crazy, but I can most definitly see a use for a bayonet on a home defense gun. The use is 'intimidation'. It would be very handy to keep a perp at bay and subdued while the Police arrive, without having to dirty up your walls/floors/ceilings with blood/brains/guts. And yes a knife on a stick actually has a psychological intimidation factor. Alot of people that will 'try' somebody with a gun will back down from someone with a knife on the end of a stick. In some ways knives (on or off a stick) are even more intimidating than guns.
With that being said, neither my 870, nor my 590, nor my 1100, are my 'home defense' weapon, that is a pre-ban bushie M4orgery with a surefire, and a bayo handy. If I had to choose a between a 870 or 590, it would be the 590.