Creaming should help dissolve the sugar. I wouldn't use powdered because it's been ground so fine the amount needed would differ and it's quite complicated to work out amounts. If you really dislike granulated try castor sugar which is a finer grind than granulated but the amount needed will be the same
Doesn't powdered sugar have corn starch added to it? Or is that what makes confectioners or icing sugar? I have always understood that these are used interchangeably and are all the same thing, just depends on where you are from...sort of like fondant and sugarpaste, same thing, different names.