Hi Maxsibs,
Greetings!!!
For now there is not any such out of the box solution available to update the default quantity as 1 in the document lines, whenever the user select Type as Fixed Asset.
A small customized enhancement is required to be developed on the related documents to update the quantity as 1, whenever the user select type as fixed asset in the lines of the related documents.
I hope it will help you in resolving your concern.
Best Regards,
Sheikh Muhammad Jawad