Hi Bunny1999,
You can see my thoughts about Karl and Sleeponlatex in post #4 here and post #3 here and a forum search on “sleeponlatex” will bring up more information and feedback about them as well. I would consider them to be a reliable supplier of latex components and toppers as well.
Yes every change in a layer or components can have an effect on the feel and performance of the other layers and components and the mattress “as a whole” so a padded cover will certainly have some effect on the feel of the latex below it. You can read more about quilted covers vs non quilted covers in post #12 here
Phoenix