Yes it is acceptable to use stainless steel staples for fixing these boards. The specification for the staples for fixing PROMATECT®-250 and PROMATECT®-XW boards is: 35mm long x 12mm wide x 1.6mm thick staples for boards up to 15mm thick, and 50mm long x 12mm wide x 1.6mm thick staples for boards above 15mm thick. These can be sourced using the below links:

https://fixingguns.co.uk/staples/im200-staples 

https://pronailers.co.uk/products/16-gauge-heavy-duty-medium-crown-stainless-steel-framing-staples-32mm-50mm-2-000-box-2-fuel-cells? variant=39516777447560