I am making a wedding cake and some of the guest are diebetics. So i am making a sepreate cake for them. I have the cake recipe now i am looking for a good icing. They want the icing white,and not to sweet. Its going to be outside and its going to be hot. This is going to be 2 8" circle white cakes.

May I suggest you use the Pillsbury sugar-free that comes in cans. It has the common sugar substitutes like maltose (may give you diarrhea) but it is a diabetic formula.

you can make Swiss meringue butter cream with stevia which has no glycemic effect. You can find it at the grocery store under the name of Truvia.
You can also go to a health food store and buy coconut sugar and substitute it for sugar for equal value and it too has no glycemic effect.