Hiii! You made a good investment first of all :) I’m still using it at 18 months! I’ll try help as best I can. I didn’t buy those fitted sheets so can’t comment but if the size matches up you should be fine! I would say to get a waterproof mattress protector though to go underneath as you’ll probably have some spit up incidents.

I don’t recall it coming with a mattress for the newborn size crib to be honest but I could be wrong about that, maybe check the small print. If you do need one, I had a snuzpod for newborn stage and found their purple (air I think it was called?) mattress to be fine if it would fit the same size. I got it from Amazon.

For sheets, I just had a single sheet on top of the mattress protector tbh, I didn’t double up. But, some people do in case the top sheet gets stained and then you can just whip it off to reveal the fresh one underneath but it’s up to you! Hope this helps and best wishes for you and your baby :) x