Is the Slim rust sealer pre-thinned?/Do I get more product if I buy the Original instead and thin it?

The Slim is not pre-thinned, no. The Original contains a thickener, which makes the consistency better for brushing. That is the only difference and the solid content of the two are the same, and produce the same exact long lasting results. So also no, you won't get more product if you buy the Original and thin it.