Hi barneycalhoun,
Welcome to the Mattress Forum! 
The blended Talalay Latex from Talalay Global (formerly Latex International) is 70/30, SBR/NR.
The toppers from mattresstopper.com all come with their own covering, so as you stated you’d need to one larger covering to encase your total mattress. These are generally used on top of an existing mattress, but some people do like to have their individual foam layers individually wrapped in a component mattress.
That’s one of the nice things about a component system – the ability to customize and experiment with different feels.
Some of the better options of which I am aware for toppers and component latex foam pieces are located in the component post here.
Phoenix