could this belt be used to polish after a 400 grit silicon carbide belt?

I would typically move to a 600-grit silicon carbide to keep the resin belt from wearing too quickly.

This will not "polish" the glass as cerium is the only step that will do that. 

The resin diamond belts will leave a more aggressive surface on the glass than a silicon carbide belt. You can learn about abrasive differences in our Coldworking 101 Video.

The 1200-grit resin belt is best for lapidary use and less for glass. From a silicon carbide, I would recommend a Cork or Trizact belt and then a felt with cerium for a polish.