What type of swimming would be best to tone my stomach?

No swimming stroke really works the stomach, butterfly puts some strain on the stomach but not much. Otherwise I would say breaststroke but you'll get better results in your arms which obviously isn't your target area here. But as for your shoulders they will be broader, that's just the effects of swimming and constant shoulder usage. All swimming strokes do this and you will very likely develop broader shoulders if you swim competitively.

if you are just swimming casually (2-3times a week 1hr or so at a time) you shouldn't see to much difference in your shoulders unless you are swimming really hard and pushing your limits. As far as abs.... Fly kick on your back really works the abs. For more intesity sit halfway up and fly kick on your back while still maintaining a streamlined postition.

butterfly sort of works your abbs but the best way is to do dryland an ddo things like crunches, sit-ups, and leg lifts

i dont do many core workouts at all...all i do is run a lot and swim a lot...and i must say, i have pretty toned abs...all i swim is freestyle, but i also work very hard at what i do...i swam about 8000 yards every day through the season...so freestyle helped me get mad abs, but idk if more casual swimming will have the same effect