I am trying to create a receipe that is less sweet to the usual buttercream receipe's that are found on CC. The problem is, when you use less icing sugar, the shape doesn't hold as the ratio of the fat is higher. What can I use to compensate too much icing sugar? Could I use cornflour or will that go mouldy? Any suggestions?

Honestly since icing is mostly sugar and fat I've never seen where adding more sugar makes it any sweeter than it already is?

No, don't lessen the amount of sugar in your recipe. You can alter how sweet the icing tastes by adding salt. I've accidentally overdosed a batch of icing with so much salt that it had virtually no taste at all. I recommend dissolving your salt in your liquids or extracts before adding it to your recipe. The only thing cornstarch will do is bind up the liquids, making the icing stiffer. Play around with the salt content of your recipe until you find the balance that you like.

Also, Swiss and Italian meringue buttercreams are wonderfully rich, buttery icings that are not knock-you-over sweet like the standard shortening/sugar American buttercreams tend to be.

Thanks for your tips, do you have any swiss / italian buttercream receipe's you can give me?
Also, i have already done inydebbi's buttercream, is it too late to add salt as everything is mixed?

I don't know, as I haven't made her icing. I think Dream Whip is a disgusting product so I don't use any icings that call for it. All you can do it try!

Look in the recipe section here for the meringue buttercream recipes. They are pretty standard. Italian meringue is my preference.