I don't know why you could not use swiss buttercream because that is very good icing.
Just for another option you could try this icing in the future too. This is another recipe I found on line for Flour Icing which tastes very good on a cake and it is not as sweet as the regular buttercream icing. I have tried it and I did pipe borders with it and flat flowers like daisies with leaves.
Begin by pouring 5 tablespoons of flour into 1 cup of milk. I used whole milk because I can if I want.
After adding the flour, whisk it together until combined, then place the saucepan over medium heat. Whisk it as it heats up and thickens, and stop when the mixture is very thick.
Cool the mixture completely.
The mixture will be very, very thick.
Add 1 teaspoon of vanilla to the cooled flour/milk mixture.
Stir it around to combine.
Pour one cup of regular sugar into a bowl with one cup of butter. I am not sure if you can use 10xsugar with this recipe. I tried it with regular sugar and it worked fine.
Whip it around until it’s light and fluffy, scraping the bowl once or twice to make sure it’s all mixed well.
Add the cooled flour/milk/vanilla mixture. Make sure it’s not the least bit warm!
Next, just whip it on medium-high until the mixture resembled whipped cream.
This was slightly tricky for me, as I was afraid of overbeating the mixture and causing everything to start breaking down and falling apart. I’d say this is about 45 seconds of pretty hard beating.
This was spread with a dinner knife. The frosting definitely has a whipped cream consistency; it’s not thick like a decorator icing. It can be used to make borders or flat flowers such as daisies.
Please post a picture. We would love to see your cupakes!!! Good luck, I am sure which ever icing you try will taste great!!!!