Modified seems to work fine with foster slugs, but most ppl do recommend cylinder or improved cylinder. Do NOT shoot sabots from a smoothbore unless you like keyholes scattered across the broadside of a barn (and possibly a bulged barrel if you shoot a choke). And also no shots much over 50 yards. 100 is already WAY out there for smoothbore slug accuracy, 40 yards would be an even better limit.
Get a rifled barrel if you can and use sabot slugs. Accuracy and range is much better once you find the right sabot that shoots well with the rifled barrel.