Can a person that sails a 200 ton yacht sail a ship?
You mean a 200 ton mega-yacht? For both, if they have the requisite Certificate of Competency to do so then yes. If not, then no.
There is a CoC for Master of 200t mega-yachts Master of Yachts 200ton (Limited) MCA approved IYT Captain / First Mate Course with Superyacht Crew Academy, which is not the same as a CoC for an ocean-going ship such as a tanker, container or cruise ship under STCW - STCW Convention - Wikipedia
Whilst STCW also describes certification for large yachts, it's not the same as for a "foreign-going" ship. Once a large yacht gets to the size or passenger carrying capacity of a mini cruise ship, you would have to go for a higher certificate.
CoC - Certificate of Competency.