Can you advise if using a mattress protector on safe cot mattress is okay? I couldn’t find any articles related to cot mattress protectors on the website, and I am not sure if I should put one on the cot mattress or not. How can I prevent soiled mattress if a mattress protector can’t/shouldn’t be used?

Red Nose recommends that baby be placed on a mattress that is firm, clean, well-fitting & flat.
It is important that the mattress protector does not compromise the firmness of the mattress.
.....always make sure that the waterproof mattress protector is strong and a tight fit.
Never put soft bedding under the bottom sheet, such as a sheepskin, as this makes the sleeping surface too soft.
https://rednose.org.au/article/what-is-a-safe-mattress