The platoon sergeant would be the highest ranking noncom in the platoon. So he would be directly assistant the platoon commander. So he would not be necessarily a squad leader, but in some cases, he might. I believe the Marines' platoons are similar to the US Army, with around 40-45 men (soon women as well) broken up into four squads. You can ask directly one of the Marines units Facebook pages. They would be glad to answer.
OK, I work part time as a manager, in a sales company that give homeless people income, we give them stuff to sell, the ex colonel guy, that owns it, organize them using army system and got 3 squad per platoon, so I always though its 3 squad in army