shortening: you can use shortening in the frosting, but the flavor is far better with butter. If you want to make the frosting smoother or silkier add in 2 tablespoons of shortening if the frosting gets grainy if it's over whipped (the addition of shortening will keep the frosting really smooth for the frosting tip used for the mini roses)
to thicken frosting: add powdered sugar a tablespoon at a time until the frosting doesn't "flop" or move once it's piped, to test this use a spoon and dip it in the frosting and as you pull it out, if the frosting is really soft, add a tablespoon of powdered sugar and test again. If you fill your piping bag and begin piping (test on a plate) remove the frosting and add powdered sugar and test again.